One thing I did, which really helped, was write on a load of lollipop sticks ALL potential activities at home and nearby. So when I'm loosendish I can take one out at random for inspiration.
Examples include : sandpit dinosaurs, making dens, sofa cushion soft play, tea party, train track, Lego, local soft play, cafe, park, park#2, cathedral, pet shop, walk, football, swimming, library, fish shop, making cakes, decorating biscuits, play dough, painting, museum, bus trip, watch the trains....
Etc
If you have a long think and just write them all down you can look back at the list when you're in need :)