Playtime?

What do you do with a four month old at 10:30pm when you have fed him, bathed him, massaged him, changed him, sung to him, played with him, taken him outside, lain him on the floor to play with dangling objects, done keep fit with him as a weight, etc. etc. and he still won't sleep?! There's only so much socialising with a child I am capable of in one evening!