Isabel's apartment is a great place for a young couple to start exploring Lisbon, even though the orientation is a bit tricky on the first day. But once you stick to the main roads, you can easily walk to the city center and many of the historic places of Lisbon.
Even though the center of Lisbon is quite touristic, there are many nice spots to explore around this part of town.
Nannarella around the corner has the best ice cream, Manteigaria has the best pasteis de nata, both places are highly addictive :-)