Per a local diving instructor and a friend who used to live in Costa Rica, the best diving hands down is Caño Island. I was in Costa Rica earlier this year (late January), but didn't dive. Conditions/location (Guanacaste) just were not worth it.
In general, I really liked Guanacaste. There's a reason why hotels are invested there. I do recommend going to Manuel Antonio. Amazing beaches. However, for some reason (I'm no geologist), the beaches are rather poor. We also stayed in Playa Flamingo which had nice beaches around.
Just a general description of our trip. As we arrived late in San Jose, we spent 2 nights there just to see the capital city, staying in the historic center. I'd skip that. Fly into Liberia if you can. Spend 2 nights, 1 full day in Monte Verde and/or La Fortuna. Focus on the beaches as much as possible. The ocean was unusually warm when I was there. You couldn't cool off swimming in Manuel Antonio the water was so warm.
I'm happy to speak with you on the phone if that is easier for you to help plan your trip. I can also put you in contact with a scuba diving instructor that lives in San Jose.
Definitely take your radar detector with you. The speed limits are ridiculous. The locals ignore them, so why should you? You just need to make sure you don't get a ticket as you'll probably have to bribe your way out of it.
