If you went the water filled route, most small tractors with water filled tires use windshield washer fluid instead of straight water. It's really not worth the rest of the opitons unless you have a friend who has some left over 'rimguard' or something.
http://www.michelinag.com/Learning-a...llasting-tires
http://www.orangetractortalks.com/20...-tire-ballast/
If you think water or sand would weigh too much, maybe look into airsoft BBs.