I’m looking for a great vacation spot for December ?

My boyfriend and I are trying to plan a trip. I’m 17, he’s 18. We were thinking a cruise but you need an adult 21+ to accompany you. Then we thought about flying somewhere, but when we get there we might have trouble renting a car since in most places you have to be 21 to do that. We just want to get away, have some fun, and not spend way to much money. We’re willing to spend, but not thousands, lol. Help US PLEASEEEEE ! Any ideas, suggestions, anything will help!