Our grandchildren that age loved to play in sandboxes. We filled a low Rubbermaid box with sand and sat it on a plastic tablecloth. The tablecloth was to make it possible to pick up the spilled sand. Lots of odds and ends to use as sandbox toys. The second time we stopped at Dollar Tree and got some toys instead of searching thru the TT for things that would work as toys.
Not only did our grandchildren love the sandbox buy other children kept showing up to play with them. Had to make a rule that they could not just drop over without letting their parents know.