How to be fit over summer??? No rude comments please?

Ok so im 14 years old and i was in athletics this past year and i got into really good shape. Now that its summer im not with my coaches anymore and its too hot (105) in texas to run outside. The only gym that is around here you have to be 16 to work out in. I am 5'1 and weigh 110 lbs. but i am not fat. Im muscular. I have NO idea what to do over summer. Im going on a cruise in a month and i want to look fit. I have a 2pack, kinda pathetic but at least im honest. I want a flat stomach. please help me.