I'm leaving in 24 hours and still struggling on what to pack. My itinerary is as follows:

Day 1 Arrive San Jose at noon. Leisure for the whole day.
Day 2 Highlight Tour (4x1)
Day 3 Arenal by night including Tobacon hot spring. Overnight at Arenal Paraiso.
Day 4 May be Canopy tour or hanging bridge.
Day 5 Back to San Jose. Leisure for the day.
Day 6 Relax in San Jose. May be shopping.
Day 7 Go to Tortuguero with Fran & Modesto. Overnight at Laguna Lodge.
Day 8 Back to San Jose. Leisure evening.
Day 9 Depart

I'm not going to any beach or rafting. Are shorts and sandals still necessary?
How heavy is the rain? Is it okay to bring only umbrella but no poncho?
What kind of sweater/jacket should I bring? Is a very thin windbreaker (only slightly waterproof)good enough?
Is it really unwise to bring jeans?
Your advice is greatly appreciated.

Okay, second try to get this to post...

I recently returned from CR, visiting the exact same area you're going to. We even did the tour with the Watsons and stayed at Laguna Lodge for two nights!

You will love the Tortuguero trip - the Watsons are wonderful. On the boat over, it poured {the next day was beautiful and dry}. The Watsons provide heavy-duty ponchos, so you don't need to bring them. I would wear sandles on the boat, as your feet might get quite wet if it rains.

We packed three pairs of shoes, light quick-dry -not! hikers, heavy-duty sandles, and flip-flops. This was perfect. You might get by with just hikers or sneakers and a pair of sandles, depending on your activities.

If you have gortex shoes or jackets, bring them!!! We didn't and REALLY regretted it, but then, we did a lot of hiking and tromping through rain and cloudforest.

Yes, jeans are a poor idea. they're heavy and don't dry, and could get pretty uncomfortable. I would pack shorts along with long pants. We found zip-offs a perfect solution.

A light fleece or sweatshirt is a good idea for those cool nights.

And don't forget a flashlight or two and extra batteries!

I'd take a disposable waterproof camera for those wet days & places like Tabacon, Fortuna fall, etc. Also - take lots of ziplocks & trash bags for the wet clothes/shoes. Finally, take more film than you can possibly imagine. We took our digital camera & maxed out our large memory card after 3 days - either buy another memory card or take your connector cord so you can download easily to a cd.

- Buy those cheap rain ponchos. Don't even bother with an umbrella. It will either rain in the afternoon for about 4 hours.
- Bring a lot of clothes. It's humid, yOu will probably be changing 2X a day, and clothes don't really dry fast.
- Do not bring denim, unless it's a denim skirt for you to go out in the evenings.
- Don't bother bringing a sweater jacket, unless you go out during the evenings in the Arenal area. A hoodie is fine but nothing bigger.

If you are spending time in San Jose you may want to bring jeans for those days you are in the city. The weather in San Jose is generally on the cool side and a lot of locals wear jeans. Shorts are not a good idea in the city - locals wear long pants (or skirts for women).

We visited Arenal twice in August and both times it was on a cool side so we were glad to have long pants (We usually bring a pair of zip off convertible pants and a pair of lightweight cargo khakis). I usually take 2 long sleeve tops that I can wear over t-shirts (zip up hoodies) and a cheap Walmart rain poncho. But you may be lucky with the weather too, you just never know. So bring a couple of pairs of shorts for Arenal and Tortuguero.

We brought cheap ponchos, but in three weeks, the only thing we used them for was to sit on on a boat trip when the seat got wet! They don't breathe, so they get uncomfortable. Umbrellas worked out better for us usually. The exception might be if it is cool and very rainy, or if you don't have breathable rainwear - better than nothing.