I visited years ago and loved it, it's very beautiful. The mosaic-tiled sidewalks clued me in to how artistic and well-designed the city was going to be.There's a very cool hedge maze leading into downtown, all of the neighborhoods have common and artisan restaurants and bakeries, there's an amazing waterfront track for exercising or just walking along looking at the bridge. There are several sculptures you'll come upon while strolling along the river. The museums are worth checking out, one the public food trucks near the waterfront plazas had the best hamburger I've ever eaten.

Wow, this might be that truck, Sabores Sobre Rodas, the guy was a gourmet chef for a couple decades but always wanted his own spot. If he's still there, it's right near the foot of Belem Tower, between the tower and the waterfront plaza.

There's a fun cable car, there is tons and tons of great street art to find everywhere too.

LX Factory for various neat stores and places to eat, Timeout Market for a large food court, the Oceanarium for looking at the fishes. If nerdy, then Kingpin Books and Hyper Toys. Pavilhão Chinês (bar with a fun collection of stuff). Pensão Amor is a bar in an old brothel. Guided tour at the The Banksy Museum.