Better than a city built around a highway has any right to be, in three places, and poor everywhere else.
Walking works downtown, around the Town Green, the lighthouse, the visitors centre and the ballpark, which is a genuine walking district. It works at Point Cadet once you have parked, where the harbour, the seafood museum and the schooner pier sit together. And it works all over Ocean Springs, which has the most walkable old downtown on the coast.
Walking does not work along US 90 at distance. Four signalled lanes, crossings a long way apart, no shade, and every distance walking longer than it maps. This is the single most common way a visitor loses an afternoon here.
Cycling is the surprise. The twelve-foot shared path on the Gulf side of the Biloxi Bay Bridge is flat, separated from six lanes of traffic, roughly a mile and a half with open water on both sides, and it lands you in Ocean Springs. On the mainland, the promenade and shoulder alongside US 90 give a long flat ride west toward Gulfport with the Sound on your left the whole way.
The limitation is shade, and it is absolute. There is essentially none on the bridge path or the beach road, which between June and September turns a pleasant ride into a hard one after about ten in the morning. Ride early, carry water, and get off the bridge when storms build to the north.
Accessibility. So much of this waterfront was rebuilt after 2005 to current code that the casino resorts, the Ohr-O'Keefe campus, the visitors centre and the Gulfport aquarium are level and lift-served. Beauvoir's main house is a raised cottage reached by an exterior stair, so settle that in advance. Harrison County has operated beach wheelchairs; availability is seasonal, so request a few days out. For Ship Island, describe what you need to the ferry operator before buying a ticket.