Gosh, 5 weeks feels years ago (DS is only 9 weeks now!)
We played The Fridge Game - take things out of fridge; let baby see/smell them. He did not like mustard.
Singing to him - I just googled nursery rhymes, and went through about a hundred of them.
Playing music to him. I got a good ten minutes of happiness from DS in response to Rachmaninov. I got 20 minutes in response to Westlife. Go figure.
Get him a library card? Cloth books/big picture books - of course they can't make sense of it but it keeps you talking coherently.
Trips on the bus/out for a walk. DS is fine with loud noises/lots of people so just nods off if he gets tired, but would suggest trying your DS out on a short journey first.
At 8 weeks I started taking DS swimming. Can only go in for 15 minutes at such a young age, but worth it.
Percussion set - from Mothercare, has two rattles, bells, a kazoo, etc. Wave them in his face! DS now waves them in his own face (not the kazoo, that would be pointless).
That's all I can think of ...