If possible, I'd like to gently recommend visiting her somewhere other than Okinawa. If you're planning on visiting for more than a few days (which you should), you may run out of stuff to do on island depending on time of year. I'm not sure what the USAF policy is, but Marines are able to relatively easily take leave as long as it doesn't interfere with operational requirements. A flight up from Naha on Japan's equivalent to Spirit Airlines will be cheap for her and save you the extra cost of getting a connection from Narita. There's much more to do on the mainland regardless of where you go (Tokyo, Osaka, Kyoto, wherever) and it's nicer than Oki.
If you're committed to Okinawa, a couple suggestions:
-Okuma is the USAF resort
@kinnem referred to. It's very nice but books out far in advance, especially on holiday weekends. I've never stayed there but friends did and had a great time. If you're able to, work with her to try and find a resort or AirBnB off base. The Kadena base lodging is all very nice, but stuff off base is comparatively inexpensive for amenities and it's cool to go to a different part of the island.
-What time of year are you hoping to go? The summers are very hot and humid but allow for exceptional snorkeling and scuba diving if you are at all water people. The beaches are okay for sunbathing, but swimming isn't always great: the beaches are largely coral based, and so depending on tides are very shallow with sharp coral and you have to wade out very far to get going. For other outdoor activities, there are some neat waterfall hikes in the jungle in the center of the island but they are all short/steep rather than all day activities. There are some deep-sea fishing charters that normally meet with great success as well.
-Relating to the above, if you are planning to travel to Oki during Typhoon season (late spring through late fall), PURCHASE TRAVEL INSURANCE. I am generally not a believer in travel insurance, but attempted to fly out my husband to meet in Tokyo during the one weekend that a typhoon actually hit the island. Having the flexibility to change flights without worrying about the cost being covered gave us peace of mind and saved hundreds of dollars.
-The middle of the island is dominated by US military bases (Kadena plus multiple USMC bases). This area is okay, but very Americanized. People will just speak English to you as soon as they see you, everywhere accepts credit card, and there are western food options. It's probably the only place I've visited that I didn't learn any of the language because despite my best efforts to speak terrible Japanese, people just speak English to you.
Up to the north of the island where most Americans don't really venture that is not the case. Even for a day trip or in conjunction with a hike/snorkel trip/etc, it's worth it to go up there and experience something different. Bring cash yens, though, and be prepared to work through the language barrier (most younger people speak English, with older Okinawans it's hit or miss).
-If you have time go on a battlefield tour or at least drive over to Hacksaw Ridge. There are a lot of companies that run tours of battle-sites and tunnels, mostly by retired military.
-Driving on the "wrong" side of the road is not that bad. Not sure how the USAF works it, but I had a car on my Oki "deployment" and the system for getting a license is streamlined for DoD personnel. It's not worth trying to go through the process for a vacation but if she has a car that's helpful.
-Enjoy some delicious sushi, ramen and soba washed down with Orion beer and top it off with some Blue Seal Ice Cream. Get the Blue Seal Sundae with Okinawan salt cookie ice cream and salty milk (better than it sounds, I swear) soft serve. Determine whether you are a Lawson or Family Mart family. Enjoy an adult beverage sitting by the beach at Paddler's in American Village.
If you can, the ideal time for your flight is one that gets you to Japan in the afternoon/evening. I flew commercial out to Japan via ANA (All Nippon Airlines, in conjunction with I think United). My flight left LAX in the early afternoon/late morning and after my connection through Narita got to Naha at around 2000-2100. I tried to stay up for most of the flight and racked out as soon as I got to base. This got me almost immediately on Oki time. The transition back to US Time was much, much harder and took almost a week.