I've considered this for the summer. I'm still deciding between swimming, climbing or thaiboxing. Do you know any other fun ones that might make you grow muscle?
Climbing is the One True Sport for me though. Do try it! The climber's physique is quite attractive and confidence-inspiring. There is a minimum of technique that I hope people will be aware of. I don't want to make it seem complicated though, it isn't.
In the climbing process itself, the minimum technique is this: Become efficient. Get a feel for your balance on the wall, make stable triangles on the holds to free the fourth limb to go to the next handhold/foothold, and use larger joints/muscle groups rather than smaller; Most people's instinct tells them to to pull the body up with upper-arm strength, but generally one can keep the arms straight and use legs, waist, torso, or shoulder muscles to gain elevation.
And climbing is not a trivial load on the body. One can climb injury-free, but this does require a minimum technique or form. Always warm up, always perform the proper stretches (about 8 main forms of stretches, takes 10 mins or so after practice and really kickstarts the post-exercise well-being). Listen to pain: Usually, pain comes from imbalance in the muscles, or tendons or cartilage not being ready yet. Back slightly off and learn the quite simple exercise designed to put that joint and that muscle group back into balance. Climbing stresses tendons and cartilage, which by nature strengthen and heal slower than muscle.