What to do with a 3 almost 4 year old boy?

okay, my cousin who i am 2weeks older than (we r 14) and his little 4 year old brother...well almost 4 is coming 2morro and i have 2 babysit my almost 4yo cousin, and my other cousin who im 2weeks older than may be helping. whats some stuff 2 do? we got a pool, wii, computer, tv, dogs, and a HUGE backyard (about 10 acres)!!