If I was going to walk to lunch from the aquarium I'd head across President Street, up a block to Lombard and another block or two east to Attman's Deli. Corned beef, pastrami, hot dogs wrapped in bologna. It's so, so good. A Baltimore institution that deserves its status.
I really like Blu Bambu (situated between Chipotle and PotBelly) – it's family-owned and has one other location somewhere in MD.
They have several menu options, but I always get the Asian stir-fry bar. They have tons of vegetables – salad-bar style – that you load into a small bowl. Pick your sauce at the end and they'll stirfry it up for you. (You can select a meat when you pay and that gets thrown into the wok as well.)
They recently added brown rice as an option for an additional $1. I adore the woman who manages the cash register. She always remembers me, even if it's been months since the last time I was there.
Directly across Pratt Street from the acquarium is Boheme Cafe, which is pretty good for soup and sandwich (only open M-F, though). Rosina Gourmet is also a good suggestion. A slightly longer walk to Little Italy would take you to Gia's Cafe on High Street, which is nicer but not too expensive.
Depends on what you are looking for...
Potbelly's and Chipotle are not terrible options and certainly very close.
There is Chef Paolino's in the BOA building for a quick, decent, non-commercial chain option.
In the Constellation Energy building is Miss Shirley's for a more expensive and interesting menu.