I tell my players (and all my students) to play as many sports as they possibly can. I tell them you have 4 years to do high school and that there is not a "go back" button, so try everything the school has to offer, even if you are not sure if you will like it. Go out for a play, for the musical, try new stuff.
You get 4 years to make memories, to have fun, you will remember the bus rides to a game more than you will anything else. Before you know it, you will be an old person typing away on a message board talking about the good times and bragging about fake stuff and trying to change peoples minds on politics.
I have never once made lifting mandatory and I never would. But the weight room is (almost) always open. I have never once made a summer camp mandatory.
All I ask is that during the season, you don't miss practice unless something comes up.
What they do in the off season is up to them. Work, play other sports, hit the pool, take a summer class, workout, chill, be a freaking kid. It is all gone after high school.
I have so many really good players at my school that say "Yeah, I got an offer to play at (insert name of some small lame college here) but I just want to be a regular college student so I am going to UNL or CU or whatever"