Here is an idea that I do often with the children at work!
I have done this activity many times at the school and the kids love it. All you need is a pack of Spaghetti (Tesco Value around 30p) and a pack of mini or large marshmallows (usually around a £1) and that is all. Clear the table and let them build their own structures.......they will sit for hours trust me!
Also you can combine a little mathematical talk about shapes without them realising...if your lucky :) so its educational too.
Have fun :)