I plan to do stuff with my DS that he doesn't get to do much of during term time, as other posters have said. Stuff I, as a working parent, just dont have the energy to do, moreover.
I'm hoping to teach him to swim properly (he can stay afloat and doggy paddle, and loves jumping in to water etc but he needs to learn proper swimming strokes).
Also going to teach him to ride his bike without stabilisers.
We're going to do cooking and some home-made science experiments and go on trips and do lots of outdoorsy stuff that, imo, a 6 yr old should be having fun doing (instead of being cooped up in a class room all day!).
I also help him put together a holiday scrapbook for most holidays, where he writes diary entries, draws pictures, sticks stuff in etc.
And we keep up reading stories during the holidays.
I wont be doing any formal 'homework' with him, though.