I was tempted to bring heels (low, casual heels, but heels nonetheless) because we were going to at least one nicer dinner, but I'm SO GLAD I DIDN'T. Two reasons; Costa Rica is not overly developed and many of the roads are gravel only or mud, and it's such a casual place! Even at the nicest restaurants, most people are wearing shorts and a tank top. So so so so casual.
Running shoes were key, not only for exercise or excursions, but like I said, the gravel roads (and 1 mile walk we had) were easier to walk with a sturdy shoe.
Durable sandals (my Reefs) were the best pick! I barely wore my J.Crew leather sandals because they slip so badly on mud, and I didn't want them to be filthy. BUT I'm glad I had a prettier option for nicer dinners.
I brought 3 maxi dresses - but wish I would have only brought 1. For the reasons above, walking was a pain in them since I was trying to keep them off the mud! Midi dresses and regular sundresses were a much better option. These were great for dinner, because most restaurants did not have a/c or were outside.
Shorts & Tank Tops
I brought 1 pair of athletic shorts and 2 tank tops, 1 pair of jean shorts, and mixed and matched them all. Like I said, this place is CASUAL! While I'm not really a yoga-pants-in-public kind of girl, there was no point in getting all dressed up to walk a mile in the mud/gravel to get breakfast, 1 mile back, and then put on a bathing suit for the beach.
Bring at least 2! On a trip that is primarily focused on the beach or pool, 2 bathing suits are necessary. You don't want to get stuck with a damp suit from the day before (yuck). I brought 1 cover up and bought a pretty sarong on our first day in town!
I don't burn easily, but I do wear 15-30 SPF at all times. Matt burns quickly and easily, so he opts for rash guards & sun shirts that protect his shoulders and chest from the sun. He still applies sunscreen to legs & face, but his shoulders are always what get charred. Bring lots of sunscreen, any protective clothing, and a hat! There are also a lot of bugs around, so bring a good bug spray - especially for the evening!
Costa Rica is HOT. Hot hot hot. We were visiting over Thanksgiving, and the temperatures were in the low 90's each day. The evenings were not much cooler. We were sweaty. The sun is also quite intense, even in the "winter". The rain forest is also right off the beach, so it's very humid and can be buggy. I didn't bother to blow dry my hair the entire week, because I knew it would be in a bun before long anyway, so why fight the curl? (especially with the nice salty ocean to help!)
Part of the charm of Costa Rica was the fact that it's laid back and a little "unfinished". It's so nice to be able to be so casual in a beautiful place.
I hope this helps you plan how you'll pack for your trip to Costa Rica!