Well, since you only have a couple weeks at this point, trying to increase your strength is not going to play a big role in increasing velocity. The gains you'll be able to make will be minimal, and utilizing the strength increase for pitching specific will take a while. So while the benefits are low, there are problematic aspects here as well. If you try to go hard on strength work, the resulting soreness will make it difficult to work on sports specific things like pitching, hitting, etc.
So basically I would focus all your strength and conditioning on compound (multiple muscle groups) movements. Explosiveness is the key aspect here, and volume should be low. Dont use any weight that overtax your muscles (I'd stay below 75%). Since you guys have sandbags, I'd work on some sandbag shouldering (floor to shoulder), overhead throws (sandbad to chest, then throw overhead behind you). If you have a barbell, then powercleans would be good. For dumbbells, one arm snatches are good for explosion. Also if you have any sleds (football should have some) or parachutes for running, these are good for lower body power. Box jumps and broad jumps should be included as well.
Note that I mentioned quite a few options, but volume should remain low (under 15 total sets per session) so try to keep the intensity high. Reps should be around 6-8. You can break that down whichever way you feel comfortable, either 7-8 exercises with 2 sets per ex, or only 3-4 exercises with 5-4 sets per ex. Either way, 2 days a week should be enough to maintain your strength and power without causing any fatigue that will hurt your performance. Try to avoid working out the day before a game.