Which Cruise are you going on?
If you have the option, seriously consider driving the day before, arriving leisurely (or late!) at a local hotel, then drive to the port in the morning and park in the garage there. There's no sense in being too tired to enjoy the first day (afternoon & evening) of your cruise if you can avoid it!!! With your husband's situation, you might want to clean the car of change & valuables before you leave home, and let them do Valet parking, if it's offered there. At Tampa it's literally only a few more $$ a day and they take the car at luggage drop-off and bring it right to you on your return (your final TIP of the trip!).
There's probably going to be 30-40 minutes of standing and shuffling in line that DH might have to endure. WRT early-boarding, I'd see if your TA/PVP can provide you with some kind of advance paperwork/pass/tag "authorizing" expedited checkin & early boarding (unless you have Cat 11 or 12 stateroom), I don't think "boarding" is the issue - I think avoiding standing in the check-in lines is what you'd like to bypass!!!!!
Probably your only real inconvenience will be the infamous Muster Drill where they go over evacuation procedures - your muster assembly area is clearly identified in your stateroom and there are arrows all over the place showing you the way. Most of the assembly areas are on the Lido Deck near the Lifeboats. They announce the Drill with plenty of notice - you have to scurry to your stateroom, get your lifejackets, note where your muster station is, then scurry up the stairs/elevators with the other 2000 passengers, and make your way to the Muster station, then stand for 15 or 20 minutes. The drill seems to be just as the ship is preparing to cast off or shortly after clearing the harbor, presumably so everyone can watch the ship leaving port. To avoid the cattle call, you might want to head for your stateroom when you board, drop-off your carry-on, check your Muster location, and take your lifejackets with you, then head topside for lunch and sunshine. There's always some who do this; at the cost of maybe looking a little dorky, it saves the cattle rush at the announcment!!!!!!!!! [They DO have jackets stashed on the Lido deck for emergencies and you might in advance explain to one of the crew and ask to use them]. Everyone has to participate, they lock everything up, and hunt avoiders down like dogs!!!!!
If walking is a problem, I'm not sure what excursions would be suitable. But the good news is that the ship will probably be 75% deserted and pretty much yours. Probably the formal dining room will be closed at lunch in port, but that's about all. If you want a loung chair, you need to get out there early - they go quick. Get to the shows at least 15 minutes early for a decent seat.
You're assigned the same Dining Room and the same Table and the same waiters for the trip. They wait to take orders for everyone at the table to get there, so if you're going to be late or a no-show it's good to let your waiters or your tablemates or the Maitre d' know. For Breakfast or Lunch in the Dining room they show you to a seat. Many feel breakfast is better in the Lido buffet. Note that there's a made-to-order omlet station in the Buffet for breakfast, and a made-to-order hot sandwich station at lunch - GOOD. there are something like 5 kinds of Pizza and Caesar Salad at the Pizza station. The buffet is carry a tray to various stations, then find a seat.
Since you're in the area anyway, and getting off the ship early, mid-morning you might want to stop at the nearby Kennedy Space center for the day. You can reserve tickets on-line at their website.
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