What we all know as the Nature Centre would be my first shout Here
Think Tank - Not cheap, but a lot to do and see there Here
All the Birmingham Museums and Art Galleries. They usually have events going on in the Sumer holidays BMAG . Depending on where you are coming from, some easier to get to than others.
Linked in with the Commonwealth Games, there is supposed to be something called Perry's Trail - but in truth, it is proving difficult to find out any information about it.
They are also doing something called 'Key to the City' which takes you to places you can't usually access.
West Midlands Police Museum is good here
A wonder around the canals is good - or trip on a boat if you want to
A visit to the Art Gallery is always worth it - mine used to just go and marvel at the size of some of the paintings. It is free so somewhere you can go and just do a bit of rather than trailing them round the whole place. Another week you could do the Egyptian Gallery or something
I've never been, but understand there is a BBC Museum in the Mailbox.
Cadbury World is another paid attraction - some say quite expensive for what it is but I think you can get discounts if you arrive by train (plus things like Tesco vouchers)
Near to Cadbury World, in Bournville is Selly Manor which is good - and again, often put on days specially for kids.
During the Commonwealth Games there are all sorts of things going on - 'Have a go at sports' things and big screens in the City Centre with chance to watch some of the Games with other people