I like Trill's suggestion of leaving it at your dad's - L & granddad will have a great time building things together !

In the meantime, get your hands on some duplo. You might have seen my other thread about getting rid of the little people stuff - it's being moved along to my new little nieces - and I'm basically happy with that cos we have such a great set of duplo which all packs away into the one tub but is played with endlessly. Even the primary school aged kids who visit all play with it - there's something very appealing about those big blocks. You can buy a bulk lot on ebay and then just add a few small sets for variety pieces (vehicles, people, animals, etc).