Just because you're in the centre of Portugal it doesn't mean you can't go to the beach. There's a whole network of river beaches and Ilha do Picoto in Avô is one of my favourites.
There's a small island in the middle of two rivers which means there are plenty of options for getting in the water. There are deep places for jumping off rocks or swimming and shallow ones for just splashing or sitting around.
As well as a sandy beach, there's a grassy area with a kid's paddling pool, picnic benches and shade.
It's easiest to get to if you've got a car but there are buses in the summer from Oliveira do Hospital so it is possible, if a little complicated, to get there without one. There's a campsite nearby and plenty of restaurants and cafés besides the one in the centre of the beach complex.