We were talking about this in our Nov2006 Baby Buddies thread. Soft drink bottles are a great cheap toy, fill them with shampoo mixed with glitter, or 3 parts water to 1part oil and some food colouring. you could also put rattley things in them but obviously make sure the lid cant come off in case of choking!
Also, find things in the kitchen to play with, plastic spoons/egg flips/bag clips etc, or pots and pans are noisy fun!
If he's on solids you can put different foods on his high chair tray to play with, like yogurt/jelly/cornflour mixed with water
Have you subscribed to some websites? Babycenter and Fisher Price email you with baby milestones and they often have ideas of things to do with baby.
Thats all i can think of at the moment, hope that helps!