You are visiting Lisbon but you need a break from the hustle and bustle of the city? Rent a car and drive for about an hour to Escaroupim for a boat trip. Enjoy the quiet of Tagus river, visit the Island of Herons, Island of Horses (with the much known Lusitano horses), Palhota's village (a typical stilt village Avieira) and other islands.
Don't forget the swimsuit; you might stop at an island for a quick swim before the island disappears with the incoming tide.
There is a restaurant nearby, but why not bringing some drinks, sandwiches, fruit and whatever pleases you and make a picnic. There are tables near the river.
I would suggest Rio-a-Dentro for the boat trip. The sailors will be Rui or Luis, the two brothers behind the company; they will welcome you on their boats with a smile, some info and willingness. In case you are a bird enthusiast, they offer bird watching trips. Whichever trip you choose, it’s better to book in advance.