Make a maze out of cardboard or old VHS tapes and watch him find his way through to the food.
Take a toy, tie a string on it and drag it on the floor. Entice him to chase it.
Take him out of his cage and cuddle with him. It’s simple, but it’s fun.
Give him a bath.
Build an obstacle course indoors or outdoors and watch him go through it. Make tunnels, see-saws, things he has to climb over, etc. Make it fun.
Get an old sock and play tug-of-war with him.
Get a small ball and play ball with him.
Put him in your lap and read a book out loud to him.
Get your camera or smartphone and do a photo shoot. Even more fun if you dress him up.
Shoot a video of your guinea pig, then edit it and add cool effects using video editing software.
Take some small cloths and paper tubes and hide food or other objects. Spread them out on the floor and watch your guinea pig burrow and tunnel through to find them.
Teach your guinea pig to bowl. Set up some “bowling pins” (anything small enough to knock over). Get a small ball, like a ping-pong ball. Entice him to push the ball to knock down the pins. You might need to use treats to encourage him.
Organize a guinea pig get-together. Find guinea pig owners in your area, arrange a meetup, and socialize. Let your guinea pig meet someone new. (don’t put a female with a male, though!). Post an ad at meetup.com or craiglist.