Speaking of buckling up, if you have little ones, make sure you bring car seats! Even boosters for medium ones. I have heard of people getting ticketed when their children weren't appropriately secured in transit.
It really cannot be overstated. Bring good quality swimwear, masks, snorkels, and fins, if you will stay some time and would like to spend a lot of time in the water. You can't really find these things in Tamarindo/Flamingo areas.
**Bring more swimwear than you think you will need. You never want to be unprepared for an impromptu beach visit, even if you've been a couple times today already ;)
Sunscreen is SO EXPENSIVE IN GUANACASTE. Bring whatever you can - upon researching logistical and TSA legalities, of course.
If you or your kids are hooked on peanut butter, consider bringing some big tubs of it. It's inconceivably expensive in Guana.
Same goes for good quality shoes. Our kids went to school in CR and when time came to get school shoes, pickings were slim, my friends. so slim. (and expensive.) That being said, there's an awesome Havaianas store in Tamarindo ~ but sometimes flip flops don't fill the need at hand :)
Same goes for good quality clothing, hats, socks, underwear -- just bring good quality things that will last if you will be there for some months.
If you want to visit the Central Valley, make sure you bring some warmer clothes than what you'd pack for the beach. Going from Guanacaste heat to the 50s-ish fahrenheit in Monteverde area was shocking, I tell you! We even tried to find jackets beforehand (in Santa Cruz, CR) and were unsuccessful. There are clothes available in Monteverde area shops but for a premium! ouch.
For longer stays, if you're a cook, it's advisable to bring your own implements. It's just hard to find things like this in Guana. Like a good sharp knife (check me on the legality of that one!), your vitamix or ninja if you can't live without your smoothies, even your countertop mixer if you like to bake a lot.
Don't forget about prescriptions ~ plan ahead ~ get them filled in your home country if you anticipate that to be easier.
Bring a willingness to learn Spanish. It's just the right thing to do. No matter how loud you speak your own native language, remember, natives of Costa Rica are Spanish speakers. To me, this comes down to a respect issue. (personal opinion!)
Are you a cyclist? Consider paying to bring your bike with you. I would like to say, it can be dangerous to bike on busier roads with car traffic (drivers in CR can be a little anti pura vida...) but if you're going to be in a more tranquil setting, or you're just that brave - definitely bring your bike. My husband actually bought one when we were there and it broke the first day he rode it. true story.
Bring your pura vida. That means ~ slow down. take in the sights, sounds, smells, and beauty all around you. Smile at that stranger and say buenos dias (or "buenas"). Tickle a baby.