That's quite a list!
I like Tigger's idea with the camera. It would be interesting to see what happens "in there"
Another idea would be to create a painting by placing paper on the floor and moving over it with the handle of a paintbrush inserted. I've never seen that before. Might be a problem with it falling out, though.
If Rob has a pool table, you could lie on it and use a pool cue. I know, it's been done many times before, but I think it's cool.
Something that lights up would also be cool, maybe a lightstick or small flashlight?
I'd suggest avoiding anything glass unless solid for safety reasons (bottle necks are probably ok, you've proven that already
). You should also be careful with beads. The strings on cheap ones could break leaving no way to retrieve them