Like mentioned before there's praca da figueira. In the evening you can usually meet some locals there and ask for other spots. When I was there the last time it was only skateable from 6:30 pm or 7 pm on. A local told us you might get trouble with the police otherwise (he said they hand out tickets and even confiscated boards) Close to that spot there's also a skate shop called tribal urbano.
There are some ledges at praca do comercio and you also have the expo skatepark at parque das nacoes, that's the one under the bridge (vasco da gama). There's also a watergap at parque das nacoes.
In Setubal (that's near Lisbon) you have another famous spot called largo de jesus.
Alfama (the spot with white benches, the river and the red roofs in the back) which was shown in one of the videos is not skateable anymore.
Besides that, like always, keep your eyes open. We found a couple of fun spots just by walking through the city.
And when you're there make sure to spend a night out in bairo alto and go to that bakery where you get pasteis de belem.