go to your local toy library, and see if they have any larger toys like cubby houses, indoor slides etc to borrow. Best thing is, as you return them after a couple of weeks, it keeps it fresh for the kids!

See if you can clear out a room or a bit of space - even a neighbour's garage - and set it up with the big borrowed toys. We have a climbing frame thingy ($180 from myer) that's blow-up and it keeps Pip amused for hours - and physically tires him out. Also stuff like ball pits - buy a couple of hundred balls (usually heaps in second hand kids shops) and fill a porta-cot with them, give tasks like finding 5 blue balls etc.

*hug* good luck