My DH's boys are here about the same and are just a couple years older. A couple years ago I made them coupon books that they could use on their visits (tobogganing, hot chocolate, choose dinner, choose movie, bake cookies, etc) and they loved it. It cost me next to nothing too. I made 24 coupons each so they could use 2 per month and made sure there were some for each season.
another idea might be things like family board games you could play together. Walmart here has card versions of monopoly, battleship, sorry, etc for about $10.
Ive also gotten craft supplies from the dollar store and given them a "kit" so we could do crafts. I got paints, stickers,etc and some plain wooden boxes so they could decorate their own treasure chest to keep at our house. They keep their Lego men in them.