Praia de Caxias is the closest break to Lisbon city center. It only works on big winter swells coming from the south/southwest. The lefthander breaking from the jetty can be lots of fun with the occasional barrel.
On really big days, you can also surf a righthander on the other side of the bay. Not world class, but worth a try if you want to surf close to Lisbon when everywhere else is maxed out. Be prepared to share the line-up with bodyboarders!
Please note that the best time to surf this stretch of coast is October to March. In the summer there are usually no waves!