For my second solo European trip (2020), I flew TAP Air Portugal. The airline offers an option to extend your layover, a stop-over choice between Lisbon and Porto. I took advantage of this and opted for three nights in Lisbon.
Day trips I took from Lisbon include Belém and Cascais, both along the water. Belém at the mouth of the Tagus River and the Atlantic; and Cascais on Portugal’s southern coast on the Atlantic.