Depends what you like really? If you want to get wet there's this inflatable obstacle course in the water (sea) that you could try. Universal Studios, though I would only recommend it with one of those fast-track passes. The water park is usually quite full so not much fun, best to skip. Zip line? There might be an aquarium or maritime museum worth a look though I haven't visited those yet so cannot comment.

Skip the casino, boring as heck.

Personally I found the luge track to be the most fun (it almost feels like a ski trip on the way up with the chair lift).

The most must-see thing that I would recommend is the Crane Dance in the evening (you need to check the time or times beforehand). That and getting there by cable car (which you already mentioned you are doing). On the way back you can take the monorail to Vivo City (which is free on the return because you only pay when you take it TO the island) and eat at Food Republic which is right next to where you alight from the train.

P.S. How good are you two in terms of walking/hiking because there is a great 2-hr trek that will take you through some great places on the way (even up to the cable car station itself towards the end)? Just bring water+beer to cool yourself down (you are allowed to drink in public until 10:30 pm, though not on public transport).