Hey Hairy Upper Lip, I am guessing that you might be looking for cheap family fun because babies are actually a bit boring sometimes (though I would never give mine up for the world).
We've been bringing our son to ultimate frisbee (pick-up at a nearby park) since he was two months old, and he loves it. Now that he's an aggressive crawler, my husband and I sometimes have to rotate on and off the field to be sure that he doesn't get kicked in the head. Two weeks ago a bird "befriended" him, and Kenny crawled after the bird for a solid twenty minutes.
Now that its hot here in Raleigh, we've been dragging his high chair outside, filling the tray with water and letting him splash. We haven't been able to find a splash pool close to us, and we don't have a water table, so this seems like a good alternative.
I thought it would be a good idea to take him berry picking... it was not.
I also go running with him in the stroller. Every time we see a dog he wants to stop (at least, I think he does, he yells, "DAAAAAAAAAH"), so I stop and do five push ups then keep going. I've never been stronger, and its fun for him.