We have some quick dry clothing that we really like. I have a pair of shorts from REI that I wore over my bathing suit when we rafted. That material is great and dries really quickly. Some hotels will offer a laundry service if you want it, so check with your hotels. Keen's are a nice water shoe; they dry quickly. We wear those for wet areas and running shoes for dry areas. The waterfall is awesome; I wouldn't miss it. Haven't done the horseback riding there, but it sounds fun.

This isn't specific to CR but it's some packing/clothing recommendations I've made previously that might interest you. Happy trails!
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Pack really light - just a loosely-filled carry-on sized pack:
www.onebag.com
Take clothes that can be layered, including a light fleece for the highlands plus a light goretex jacket or cheapie poncho if you'll be there during the wet season; I'm not an umbrella person but some folks prefer them to a jacket or poncho. Good quality microfiber/wicking clothes are awesome - they're actually cooler than cotton in the heat, layer well, and dry in a flash so they're good in the highlands as well as the lowlands.

Some clothes with "wicking" tags are cheap knock-offs and wear like plastic. As a rule of thumb, anything that REI sells is a good brand:
www.rei.com
They're spendy but have great sales. I also like shopping at these outlets (after I've researched REI and know what brands to consider):
www.rei-outlet.com
www.campmor.com
www.sierratradingpost.com

Be sure your shoes (I take Keen Newport H20s and a pair of flipflops) are comfortable for walking and that your pack is comfortable; if you're interested in my take on a few great travel packs, let me know.

My packing list for CA is #14 in the FAQ thread of the Thorntree forum. It's served me well for many years, though I've added a tiny netbook computer since I blog and help manage other people's trips while I'm on the road. I usually spend 3-4 weeks each summer in CA but my listed would be the same for a week or 6 months:
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=828622

You might also be interested in the advice I give folks who are traveling with me when I lead groups:
http://www.fileden.com/files/2007/4/26/1017644//UPDATED%20PACKING%20NOTES.pdf

Hope that helps! Happy trails!

BTW, your last hyperlink doesn't work, hopeful.
Helpful list at LP, thanks.

I exclusively wear AFTCO Bluewater shorts and World wide sportsman NYLON shirts.put your wallet and other papers in a ziplock.I find Gore-tex pretty useless for anything but boots. the guides here mostly use ponchos. Nuff said.Vientos Frescos is a better place to enjoy waterfalls unless you want to be with 50 or more other tourists.

I would do the ziplining in Monteverde, the two biggest are here and it's marvellous through the cloud forest! I loved Selvetura.

As for horseback riding, you may want to plan it based on the weather when you get there. If the volcano is super cloudy you won't get anything out of a volcano horseback ride. We didn't see the volcano at all in Arenal, except when the clouds parted on the last morning! We hiked to the waterfalls but it was a boring hike along a gravel road, maybe you can rent bicycles?! If money is no object, a taxi would be just fine.

I like to carry a small microfiber towel to help wring out add'l moisture for handwashing. Quick dry clothes are the best, for sure. Cotton does not dry well in the tropics.