ro, good advice here, but I am surprised no one mentioned long toss. This is something you can do any where even by yourself if you have enough room.
I could throw long toss with my son until he was 14 or 15. At that point I had him take a bucket of balls out and throw the balls to me until the bucket was empty. I then placed the balls in another bucket until the supply was depleted.
Even if you do not have anyone to throw with, set up a target, start close and move back as far as you can in incremental steps until you reach the maximum distance you can hit the target. Take one more step and then keep trying to hit the target. Physics proves that the farther you can throw the faster your velocity is on the mound.
My son, now 17, can hit foul pole to foul pole on the home run fence on a good sized high school field and his velocity last year hit 90 and 91 peak. He is hoping for 92 to 93/94 this spring.
This is not entirely due to long toss, but it is a part of his routine.