At 17 weeks he won't be 'doing' much himself yet, but he will be interested in what's going on around him. Some stuff to try:
Put on some funky tunes and dance around like a loon - he can watch you from his bouncer and you can sway his arms a bit to the music, or you can carry him while you dance.
Do some baby yoga / exercises / massage with him.
Sing nursery rhymes, especially ones with actions, and repeat them often. He won't join in with the actions for a few months yet but when he does it's an absolute joy. In the meantime he'll love watching you do them.
Look at picture books together. At that age, my DD loved the cloth ones with crinkly pages.
If you can walk or get there cheaply, go to your local library. They sometimes have story time sessions, or if not the change of environment is good anyway.
Put him in a sling while you get on with the housework / gardening / whatever, and give him a running commentary on what you are doing, showing him items, naming them etc. Make it sound really exciting.