I have tons of ideas, I'll try to come back and add some wehn i have time, but for right now. consider your local library, you can just take him and let him look at all the books he wants and then ... this sounds bad, Leave them mess there... I just mean you can stack them and the librarian will reshelve them, not that you can trash the place. Also check into the special programs they might have like story time etc.
If you need 20 minutes of sanity, plop him in the tub and as long as you can see him for safety reasons that frees you up to nurse the baby or what ever you need to do. Try adding food coloring or letting him finger paint with shaving cream on the sides of the tub, Adding ice cubes would be cool for him to play with too.
Good Luck, oh and now would be a great time to make friends with any other preschool friend's moms invite them over for a playdate if he is still going part time ask if the preschool will let you volunteer, most don't mind newborns coming with.
I think being able to give him little gifts/rewards during the week, when ever he does something thoughtful might help. You could wrap up something that he hasn't seen in a while that has been buried in the bottom of the toy box or buy him some new things from the dollar store, or exchange with a friend.
I love getting my kids books on tape from the library, kept them quiet but wasn't as bad for their little brains as TV.
Anysort of art projects like was suggeted before, playdough, glue sticks and paper, scissors and toysrus ads, I like the paint by water books that Dollar General sometimes sells. Cornstarch goo is goo.
If you feel comfortable you could let him play educational games on the computer, I like fisherprice.com online games.
anyhow good luck.