Summer, the craziest season of the year, is just around the corner. I don't know about you, but all this heat seems to send my brain on vacation, and I do things I wouldn't normally consider considering. But is that so bad? Summer is about freedom, right? And letting go of inhibitions? A time for torrid, short-lived love affairs redolent of dime-store coconut?

I'm talking, of course, about investing. (What did you think I was talking about?) But the attraction is much the same. Something new and exciting catches your eye, and suddenly your gaze is riveted by the hottest piece of assets this side of The OC. It may not look like your normal fling, but you still flirt with the idea. This baby makes portfolio stalwarts like Wal-Mart (NYSE:WMT), GM (NYSE:GM), and Microsoft (NASDAQ:MSFT) look like granny in her 1920s bathing bloomers. Leave the likes of those to Mathew Emmert and his Motley Fool Income Investor. These tasty morsels offer a lot more excitement and the potential for big gains over a short time. And how much could you lose anyway?

That, at least, is the fantasy. Let's step away from the keg for a moment and look at the potential for love and loss in a few of the early season beach beauties that have turned my head for the moment.

Stem cell burns
I normally don't have much patience for penny stocks, and I'm especially wary of a company that feels the need to put a safe-harbor statement on the front of its website. But StemCells (NASDAQ:STEM) has already had one heck of a week, and I just couldn't avert my gaze. This is the wave of the future, right?

On Tuesday, the issue plumped up 30% on volume of nearly 11 million shares. That's about 20 times the normal number of stubs trading hands. The reason? Heatstroke? Hotfoot? Your guess is as good as mine. On Wednesday, the shares melted 11% to $1.85, a move blamed on anti-stem research comments by First Lady Laura Bush. Let's put the blame where it belongs: StemCells has nothing going on for investors.

The company is officially in the business of "research aimed at the development of therapies that would use stem and progenitor cells to treat, and possibly cure, human diseases and injuries." Translation: "We are conducting expensive studies and hope someday to make money on them." This is a firm that has been in the red forever -- or at least a decade -- and it still has very little to show for it. It wants to file its first Investigational New Drug Exemption next year. But let's not be uncharitable. StemCells does one thing pretty well: spend money.

For example, in 2003, President and CEO Martin McGlynn earned $580,000 in salary, bonuses, and a housing allowance -- plus 300,000 stock options -- while the VP hauled in another $210,000. That might not sound like too much, until you compare that to the firm's entire revenue stream of $275,000. Toss in $6 million for R&D and you see why this company is such a loser. Important research? Maybe, but that doesn't mean the stock is hot. Your summer investing dollars would be better spent setting up a lemonade stand.

If you want to learn to predict the weather in the stormy biotech world, here's a summer reading list:

Woo the T-Zoo?
Summer means travel, and well-known companies like (NASDAQ:PCLN) and InterActiveCorp aren't the only ones that stand to profit. Travelzoo (NASDAQ:TZOO) is a little up-and-coming online travel outfit that actually makes money off legions of other dot-com travel sites. I wrote about this upstart last week, but it's a bouncy, perky issue that begs for another look. It's been on a tear for quite a while, notching a 400% gain for the year before dropping and then cooking back up another 20% earlier this week. In May alone, volume increased 20 times the norm and the price shot up 240%.

The action has been so extreme that management recently took the rare but not unheard-of step of issuing a press release to declare, in essence, "We have no idea what's going on."

Travelzoo's main "product" is an electronic newsletter that carries advertised deals from a few hundred airlines, travel agencies, vacation-package hawkers, and others. Sure, it's got 7 million subscribers and, yes, the company has reported some pretty substantial earnings and revenue growth over the past year, with a gross margin at 97%. But in the end, it's still just a focused, updated version of the pennysaver that some of us still find on our doorsteps once a week. How much can that be worth?

Today, it's valued around $365 million. That's right, 52 clams per subscriber, or about $1.2 million per paying advertiser.

That's just crazy. And I'm not the only one who thinks so. I wrote about extensive insider sales the last time around. Since then, directors have dumped another million and a half bucks' worth. Follow those in the know and get out while you can. This fling can only end one way: badly.

Shocking comeback?
Taser International (NASDAQ:TASR) got in swimsuit shape a bit early this season, peaking back in spring at above $60 per share. It's settled down more recently, trading a little under $30 per ticket. Sure, the folks who got on the wave at the top are smarting, but Taser's 27% profit margins and incredible growth spurt -- sales up 290%, net income up 1,450% last quarter -- make this hottie worth a periodic check-in.

In case you've been living under a rock, or on some newsless tropical beach, Taser's X26 and M26 "conducted energy weapons," otherwise known as stun guns, have been selling like crazy to law enforcement agencies across the country. The stock has soared on that track record, plus big potential for more sales to the military and mere citizens: Yes, you can join the fun -- $600 can get you your own "Advanced Taser" with laser sight.

To be sure, Taser's got its problems. Like other tech companies, it's addicted to stock options and makes the bogus claim that these cost the company nothing, though they've made millionaires of over half the employees at the head office. Moreover, it often seems more interested in managing the stock price than shutting its collective yap and letting results speak for themselves. Finally, there's plenty of bad press and speculation about the real health effects of zapping people into submission. The word "torture" even gets thrown around. Some of this is deserved. After all, this is a company that features gloating video of officers needlessly "tasing" seated protesters. Do you really need to immobilize people who are already immobile?

Clearly, there is a liability issue to consider. On the other hand, Taser is encouraging and facilitating study of its own weapons' use, through data automatically collected and downloaded. You won't get that kind of objectivity from pepper spray or swinging batons, both of which were also used in a police stuggle with an ex-con in St. Paul, Minn., yesterday that ended with the suspect's death.

Despite these heavy issues, even a skeptic like me might find Taser worth the current multiple of 50 times projected earnings for the year. It has a unique and important product with plenty of room for growth. As summer heats up, Taser may yet rediscover its sizzle.

What now?
In the words of Mary Schmich -- often mistakenly attributed to Kurt Vonnegut -- "Wear sunscreen." That's sound advice, taken literally. But let's get into the metaphor for a second and maybe translate it to protect yourself. Summer is full of great and warm temptations, but if you fail to protect yourself, the burns may last a long time. If you'd like the Fool to help you with that sunscreen, I'm sure one of our crack newsletter writers would be happy to oil you up.

Fool contributor Seth Jayson has always been fascinated by facades. That's why he watches but does not own any companies mentioned. View his Fool profile here. The Motley Fool is investors writing for investors.