I did a lot of this stuff this summer with a group of my buddies. First we made a sandbag and did a lot of stuff with that. We also used a keg that we had throwing contests with and also filled with water to do some other fun stuff. We had somewhat of a sled that we would drag and bearcrawl with to see who could get further before collapsing. Pushing my truck was another good exercise. You can basically use anything heavy.
I would also suggest a full sized keg, the less water you have in there the more it will slosh around and mess with you. Just keep filling it as you get stronger. Another thing we used were large rocks. Just find a rock pick it up, or throw it.