Ft. McHenry tomorrow
Wine Market has an outdoor patio that might be reachable from the parking lot. Hull St has opened a lunch place down the street that may be accessible since it might have had to comply with new laws. Baba's Kitchen is simple, but has two outdoor tables on the sidewalk. The Hill on Charles St. is new with easy access but is not right by Ft. McHenry.
The three places I would recommend near the Fort -- the Wine Market, Pazza Luna, and Lucas -- all have entrances with a couple of steps. But with help I would think they're negotiable. The other advantage is that all three of these have easy lunchtime parking, and are away from the downtown Baltimore traffic (esp. Lombard Street, which has been gruesome with the construction).
You may want to check their websites and call ahead:
1401 E.Clement Street, Baltimore, MD 21230
921 East Fort Avenue, Baltimore, MD 21230
1230 E. Fort Avenue, Baltimore, MD 21230