J.D.
Although you said US, I love Coasta Rica and there are tons of things to do. When I was down there, we white water rafted, sat in the hot springs at the bottom of a volcano, road horseback through the rainforest, snorkeled in clear blue water, and took a sunset cruise on a ship made out of concrete. It was amazing. They have tons of things to do since they rely on eco-tourism. The canopy tour looked like a blast, but I forgot my jeans to be able to wear a harness without chaffing.
Most resorts are all inclusive and it was very affordable!