IIRC, Old Orchard Beach (Maine) has a great boardwalk. It's been a little over 10 years but my memory of it is of a slightly tacky but fun, retro beach town with a strong French-Canadian seasoning. The farther you get from the center of town the more peaceful and beautiful it is, and there is a long, broad sand beach.
Thinking of that made me wonder if there is such a thing as beach/sand wheelchairs and apparently (at least, per a quick-and-dirty Google) there is and they're not super expensive. Worth looking into if you haven't already?
OOB is probably the best easy-access sand beach in Maine. Alltrails.com has a page "best wheelchair-friendly trails in Maine"; most of the trails they mention are in gorgeous locations along the southern half of the coast. Mainebyfoot.com has a similar list but I think it's longer.
(Warning: in Maine, gorgeous locations that are easy to drive to can get very crowded in the summer, and the scenic coastal Route 1 can be jammed.)
Ferry rides from Portland to islands in Casco Bay can be fun just for the ride; day-tripper amenities once you get to your destination vary a lot from island to island. Portland has some great wheelchair-accessible trails. Becky's Diner on Commercial Street is a super-cool (IMO) local+tourist hangout. Somewhere near there is a fishermen's memorial that's worth seeing. Portland has buckets o' visitor attractions of course.
Ferry rides to islands farther north -- Vinalhaven, Swan's Island, Frenchboro, Monhegan, and Matinicus are the ones I remember -- are a bit longer and more expensive and IMO even more more worth it, but definitely check for accessibility. IIRC for Frenchboro the boat trip is the point -- it's narrated and scenic and you actually only spend a couple hours on the island itself, which is not really geared to day trippers.
Lubec and Eastport -- at opposite points of Cobscook Bay -- are I think the farthest NE you can get in the USA, have plenty of amenities + at least one iconic lighthouse, and are lovely places to spend some time. Campobello, FDR's summer home, is right across the bridge from Lubec in Canada.
Mount Desert Island/Acadia State Park = stunning but can be super crowded in summer.
^^ These are all coastal locations. Inland Maine is vast and gorgeous but I don't know what might be wheelchair accessible there. It'd definitely be worth looking up what's available in and around Moosehead Lake/Greenville and Baxter State Park/Millinocket. If you're in Maine in peak tourist season these
might be a little less crowded???
Read
Sarah Graves's "Home Repair is Homicide" series for fun murder mysteries set in Eastport and anything by
Paul Doiron (he has both full-length novels and short stories on Amazon) for mysteries set all around Maine told through the eyes of a game warden. Kate Braestrup's
Here if You Need Me is a memoir written by a warden service chaplain.
^^Anything RVNaut says about Maine trumps anything I say about it.
Closer to home (I assume you're FL natives?), you probably already know whether you like Titusville and the Canaveral National Seashore, but just in case you want another opinion: A big thumbs-up to that area for the stunning natural beauty and the fun-retro-tacky downtown (preserved b/c the town hit on hard times after the government space program shrank -- so see it quick before the private-industry space program gobbles it up). Fish tacos at Dixie Crossroads restaurant are a must, in my opinion. Great food, a moat with alligators in it, and they serve you corn fritters (like donut holes drenched in powdered sugar) as an appetizer. Eat dessert first!
PS for the historically minded --
On Merritt Island on Courtenay Parkway near the Biolab Road there is a historical marker for the Douglas Dummett plantation. What the marker does not tell you is that this plantation was one of the few in the area to survive the Indian attacks/slave uprising that launched the Second Seminole War. It survived because an enslaved person loyal to Dummett hid in the house during the attack and put out the fire as soon as the attackers left. (Pretty sure that, contrary to what the plaque says, Dummett was not there at the time.) I swear, some people don't get any credit no matter what they do!
It burned down anyway a couple years later IIRC. I went down the road once and it seems to be mostly a fishing spot now.