My 12 yo boy, 14yo daughter and I visited Sentosa Island in August 2016. We purchased 2-day fun pass (1-day USS and 1-day 20 activities).
Universal Studio Singapore (USS) - If you have visited other Universal Studio in the US, you can skip this attraction. Since my kids have never been to one, they thoroughly enjoyed it.
20 activities in one day - we accomplished the following (and not):
Trick Eye Museum (really fun)
Segway (too short, but it was good; we did opt for the short upgrade)
Butterfly park (a bit run down, but good)
Parajump (good, but short; would have love to do more activities offered at the Adventure land)
Luge (fun but short)
Wings of Time (nice way to wrap up the night)
Could do without:
3- 4D adventure (they were OK; wouldn't have done if not part of the package)
Fort Combat (neh)
Cable Car (It was OK, I guess)
Did not get to:
S.E.A. Aquarium (closed early due to private event) - I made a fuss at the ticket counters (both at SEA and Sentosa Island) about this one. All the attractions are independently operated. When we picked up our pass at the Sentosa Island ticket counter, they didn't communicate to us with regards attractions that are closed or any that are closing early. When we got to the SEA Aquarium, expecting that we have an hour to visit attraction, yet to learn that it would be closed in 3 minutes. SEA Aquarium is the next major attraction after Universal Studio on the Island, so we were very upset when we were told that we couldn't visit. Same thing happen to an Australian family and they were very upset about it. We were insistence in returning to see the attraction that they eventually relented (after several phone call of staff with their manager etc), though it took over an hour of our persistence and waiting which also prevented us from seeing other exhibits. It is certainly a must see of the 20 activities.
Flying Trapeze (closed) - The Australian family was very upset over this as this was their daughter's must do attraction
MOSH (heard that it is for pre-schooler and not to bother)
Catch a wave/Kayayk
We accomplished 11 (+SEA aquarium on a different day). I guess it is still good value. The experience with SEA Aquarium closing early had dampened our enthusiasm of the island. If you decide to chose the 3 or 5 activities option and had never done any of them, these would be my must do:
Wings of Time
Trick eye Museum
Others would be up to you, as we did not do some of the activities.