Sometimes fun can be as simple as climbing onto kid sized stuff while visiting with family (the fun of which is apparently amplified by only wearing your diaper and a pair of socks)...
or climbing onto adult sized stuff and sitting at the table like a big boy. Add some cheerios and a grandma and grandpa and it's downright delightful.
Sometimes fun is hanging out with daddy and watching the game (while my chair is rediculously close to the television).
And sometimes it's diving into drawers and taking everything out that his little 18 month old body can lift or throw.
whatever toots your horn little buddy.