when you say optional AI, you mean you don't want the AI, correct?
That leaves the Islander and the Caribbean. In that case, the specific one into which they trade is irrelevant - the two are interconnected and they can and should use the facilities of both. You should request and/or look for both online, that doubles your possibilities of getting one. We own at both and often trade in for extra weeks, and have zero preference in terms of which we get as our trade.
The villas are identical, differing only in decor. All villas have at least an ocean view; the higher up the better view, and if they are lucky enough to get an ocean front exchange, then that's all they'll see. With an exchange, the location is of course pot luck, depending entirely on what locations an owner threw into the pot.
In terms of restaurants in walking distance, only Captain's Cove, which is owned by the Royals and is across the street from the Islander/Caribbean. Very nice seafood and if they get seated right on the water (lagoon, not sea), they can watch the resident crocodiles.
There are also four restaurants on-site: Tradewinds in the middle of the Caribbean, and Paco's Tacos, Cayo Largo, and El Conquistador in the middle of the Islander. El Conquistador is one of Cancun's best restaurants, pricier and dressier than the others, the type of place for a special dinner out. Paco's is a lower-cost casual Mexican place, and Tradewinds and Cayo Largo have a mix of seafood, steak, pasta, and so forth - nothing particularly memorable but ok. There are also pool and beach bars with food service.
There are many many restaurants off-site, all price ranges, all types of cuisine, clubs, music, etc etc.
PS - regarding the Cancun (formerly VCI and Club Internacional), I would probably not recommend it for this age group. The water is very shallow and flat, great for a baby or toddler, but no waves. The resort is quite peaceful and quiet - perhaps not what they're looking for.
PPS - If there's any way they could rent from an owner rather than doing an exchange, I think they'd probably enjoy the Sands more. It has a younger more active atmosphere. As suggested above by JBuzzy11, they can go to the Sands and the Cancun, and there is a free shuttle (only hourly between the Caribbean/Islander and Sands, and less than that to the Cancun).