If you have some time free while visiting Lisbon, one thing you can consider doing is taking a boat trip along Tagus River. Renting a boat with skipper is common in South Europe, I've seen quite a lot of boats for rent in Napoli or Croatia too. It should be more fun in the summer, as you can find some nice spot to swim.
I found the boat on Airbnb experience, there should be more local boat renting website, tours on Airbnb are mostly 2 hour sunset/after trips along Tagus River, in some other websites should be able to find more longer day trips, especially in the summer months. Sunset boats cost €20-30 pp depends on the guide themselves (whether drinks and snacks are included), if you are lucky you can have the boat for yourself and your group. My guide Teresa and her crew are friendly and nice, they provided wine and hams, so it was a nice trip.
Here is the vlog I made:
Back to the deck, finishing the trip.