Both are fab cities!
Lisbon -
Walk loads, go on the tram, go to Belem to the Pastel de Nata cafe and all the lovely buildings on the river front
Get the train to CAscais and spend the day by the sea
Get the train to Sintra
Go to the Time Out food place
Porto -
Tram to Foz and have a day walking along the sea, stopping at beach bars and cafes
A Port Tour (we did Taylors - was really good)
River trip under the 7 (?) bridges
Walk loads, and then walk more! See all the tiled churches etc
Go and look in the station (if you're not arriving by train anyway) - it's got an amazing foyer
Enjoy the views from high up - climb one of the towers
The Harry Potter bookshop, if that's your thing...
Plan where you might want to eat - lots of places get booked up in advance, the best ones are tiny and busy.
Marvel at how nice (and cheap) the wine is.
Have a Porto Tonico. And have it early in your trip, so you've got time to have more!
We didn't do them in the same trip, but absolutely loved both.