BIL bought the boys a bubble machine!!! It's battery operated (there's a power lead too) and you fill it up with bubble juice and then it blows massive amounts of bubbles.
I just put it outside on the boy's playgym and for as long as it blows bubbles they will chase them - they're crazy about it and it keeps them running around for ages.
I love it too - having a garden full of bubbles is soooo beoootiful!!!