Here's some options. I've been to all of them if you have questions. I included brief +/-'s based on your posts.
Grand Cayman -
https://www.caymanislands.ky/ -
Cayman Islands Tourism Association - Cayman Islands Vacation, Grand Cayman Tours
Seven Mile Beach is an excellent beach. Resorts/condos lining it. Some like the Marriott/Westin have dive operators on site - all the others will pick you up anywhere along it - there's about 2 dozen dive operators. Also there's snorkel only operators - often snorkeling near Stingray City at Sandbar - 4' deep. Diver operators also take snorkelers for a fee - the dive is only 15' deep. The Kittiwake wreck is both a dive and a snorkel intentionally. Turtle Reef on NW point is listed as both on Dive365cayman.com And it is, I've been there. Sundivers on-site can hook you up with a dive guide. Red Sail at the Westin does afternoon dives where they invite snorkelers - shallow location, probably discover divers/cert dives also. There's nightlife and dancing on Grand Cayman - maybe 1/2 dozen clubs or so. O Bar is one I remember, there's also Jet and others. Margaritaville downtown for the cruise tourists. Which there's a lot of daily. Parts of Seven Mile Beach can be a little congested also - esp. when the cruise ships are in since they bus them to the beach and the tourist attractions past it.
Curacao -
Curacao, a unique Caribbean island paradise. Official Curacao site
Lots of nice beaches, most are also shore dives. Most also have an on-site dive operator. At some it's fairly easy to find a buddy - we were asked twice at Porto Mari - a locals beach. Also boat dive operators work the entire west coast. Ocean Encounters welcomes snorkelers on their afternoon boat also. Plus there you've got the Dive Bus - escorted diving is all they do plus they'll pick you up most places in town. Snorkelers can ride along for a reduced fee. Contact Bas Harts Diving also - he can probably do sites good for both. Playa Jeremi - ironically one of few sites with no facilities - is on Frommers Top 10 Caribbean Snorkels list. There's also a combo dive/snorkel/beach boat that goes out to Klein Bonaire a couple days a week - haven't been on it so IDK details.
In town you'll find nightlife, dance clubs used to be around Salinja. More than once driving around at night we heard one near downtown also. Mambo Beach Club is both - a DJ'd club and a beach bar. There's also jazz at the Avila. About a dozen casinos at the resorts also.
One interesting option for Curacao would be to spend your 2nd week on Aruba. Lots of nightlife there and some decent boat diving. The Antilla is a pretty famous wreck. IDK Aruba except that it's pretty cheap to fly locally between the islands - one of the US carriers lands there and people move over to Bonaire and Curacao the same day easily. Midweek I've seen prices around $100 r/t.
Last option is Turks & Caicos.
Beautiful by Nature - Turks and Caicos Islands - Turks and Caicos Tourist Board
It's much like a smaller version of Grand Cayman with an even better beach. Grace Bay Beach is named one of the nicest inthe world. Lots of upscale resorts border it. Diving there is off Grace Bay or NW Pioint by boat. I don't think there's a lot of snorkel/dive combos but there is snorkel operators. The best of the diving is far south though off West Caicos and French Cay - the moorings are too deep for snorkelers and the trips run all day basically - pickup at your resort between 8-9AM return at 2:30PM. I know there's some nightlife/dancing in Grace Bay - I was at one club there that did with a live band on Friday night. And heard at least one more driving around at night. Also two smallish casinos. They offer free pickups if you want to enjoy your adult beverages. If you have to bring the kids, Beaches there would be an excellent option - they have programs for all ages. And advertise they'll take them away so you can be alone.
I realize Oasis's business is about booking them but DO NOT take your g/f on a liveaboard - guaranteed she'll be bored the 2nd day. First of all everybody is a diver. So the boat empties 4-5x a day. Then when the divers are back all conversation at lunch/dinner is typically about diving. Followed by hours at night looking at photos/videos shot that day. Then everybody is tired and goes to bed about 9PM. I'm an insomniac so I spend a lot of hours on the boat alone - occasionally run into the night crewman on watch.
Plus it's not a cruise ship. Meals run around the diving, if you're lucky there's a hot tub on the sun deck. A couple have kayaks or one even has Hobie cats. Even the boats that offer shore excursions may be more limited than they sound. The best boat for that is probably Cuan Law in the BVI's but it's not the best diving and they only port 2-3 times during the week. There's some really nice deserted beaches I suppose the g/f could kayak to. It's all pretty shallow so there is snorkeling options also. It's just that she'd really be tied to the boat and the dive schedule. Since there's probably not enough extra crew to take her somewhere - when they port they tender people on shore using the dive tenders which are obviously in use all day during the dive schedule.
Really I wouldn't though if this trip is supposed to be about her. Two of you sitting on deck while others dive makes no sense. And they're not large ships - usually around 100' or so - so there's not even a lot of privacy. And the walls are pretty thin...lol.
