Mesa Ventilator Mid
And take this: An umbrella like GoLite's Dome Umbrella ($20; www.golite.com). Lightweight, really breathable rain projection!
I'd take the same approach to footwear. All of Costa Rica is not a swamp, of course, but you'll still be hard-pressed to keep your feet dry—inevitably, you'll over-top the boots with water. So waterproof boots will just make your feet sweat more. Maybe take a couple of things, if budget and packing space allows. Merrell's Mesa Ventilator Mids ($80; www.merrell.com) are lightweight, breathable boots that should hold up pretty well in a damp environment. Salomon's Tech Amphibs ($70; www.salomonoutdoor.com) are water shoes designed to get dunked, then drain out water.
At the same time, check with your host/guide/tour leader. Snakes—among them, the deadly, six-foot fer-de-lance—are common in Costa Rica, and you might be better advised to wear tall boots such as military-style combat boots. Better snakebite protection.
Don't mean to alarm you! It'll be fun, regardless, but hopefully a combination of the above options will cover you in your rainforest trails.
For more lightweight, waterproof footwear options, read "Make a Splash" from the July '04 issue of Outside.