Costa Rica and Belize are two of my favorite places. I think you would have a great time in either area.
Costa Rica is the pricier of the two countries. The highlight for many is the Tabacon Resort, a place where you can sit in hot springs while watching an active volcano. You'll need a car to ideally see the country, but this will free you to visit out-of-the-way fincas where you can have some amazing adventures, like horseback riding to bat caves. Manuel Antonio on the west coast is a magical park, where monkeys and iguanas surround you on the beach. The country also offers some interesting rainforest canopy tours, and whitewater rafting is available. You can have fun practicing your Spanish.
On the other hand, the most relaxing vacation of my life was on Caye Caulker in Belize. The only time anyone is ever in a hurry there is to get to happy hour at the Sand Box, the favorite bar/restaurant. You can snorkel with sting rays and nurse sharks in warm clear water. Belize offers wonderful diving and has the rainforest too. English is spoken and U.S. dollars are accepted.
I love both spots. But I'm planning to return to Belize.