Birthday party ideas

We're doing our first born his first ever birthday party for his 5th birthday! We've hired a hall and are booking a bouncy castle, I'm just wondering what other activities to put on too. I'm wanting things that are fairly cheap and easy to organise, and that the kids and do on their own. I'm going to be 7 months pregnant, so the easier the activity the better! There's going to be a maximum of 20 children, although I imagine some he invites won't be able to attend. Thanks for any ideas