Any fun (free!) ideas for a day of babysitting 2 boys aged 5 & 7?

I am looking after my nephew and my step nephew next week 8am until 6pm so a looooong day! Any ideas for fun things to do to keep them occupied and also give them a really fun day. On a tight budget so its got to be free or close enough. Ideally things that can be done at home and in the garden.

Thanks