Costa Rica is only ten degrees North of the equator. Thus, the beaches of Costa Rica are hot and humid. You will need a bathing suit(s), cover ups and sandals or flip-flops. You will definitely need sun screen. Don't think for a moment that those couple of hours you spent at the tanning salon can prepare you for the sun here. The UV factor here is NEVER below 10. Bring mosquito repellent as the beaches are buggy and you need daytime protection from Dengue Fever
If you plan to shop at the numerous beach stores and markets or just walk around, you will need very light clothing. Most shops are not air-conditioned. The nights are warm and humid, with temps that seldom drop below 73 degrees F but may seem cool after a long day in the sand. Many tourist areas have night spots, but mostly you see informal dress. Pretty much nobody dresses for the evening at the beaches! But, just in case, bring something a little better. Bring something to cover your head during the day.
In many places, there is dancing! So bring whatever footwear you need. Bands are live and loud! Dress is all over the place, but many Ticos, especially the women, use this time to show off their.... umm... best features.