There are unlimited games and ways to roughhouse, from good old-fashioned wrestling, to more structured games. The key is of the adult to keep a playful attitude, maintain a challenging but safe environment, and ultimately let your child overpower you.
Here are some ideas for starters:
Sock or snowball fight
Stay on the bed (Your child tries to escape form the bed and you try to keep them on.)
Blocking (ie, "I bet you can't get past me into this room!")
Horsey rides or bucking bronco