HHN is the best there but other than that make sure you do Cylon, Mummy, Sesame Street if you like quirky elmo vibes, Transformers if you’ve never done it, and check out the Shrek area if you love Shrek (the attractions themselves are ok but if you have nostalgia for the series you’ll enjoy them). Didn’t get to do the JP rapids when I went but that’s probably worth doing. Lights, camera, action can be cool if you haven’t seen any Universal practical effect show and Waterworld if it’s running is definitely a must see show at any park with it if you haven’t done it.
In terms of the city, definitely go to a Hawker center and try black carrot cake (it’s not actual cake and it’s amazing) and food in general, definitely go to all the main neighborhoods sites tell you to go to, Haw Par Villa is a unique place if you have time for it, and the ArtScience museum is cool as well as the National Gallery (I really liked art museums there in general). Don’t waste much time at Sentosa tbh. Out of the major tourist attractions, Gardens by the Bay is probably the most interesting, don’t bother spending too much time on the light show outside it or that area near it in general. If you don’t feel the need to see a huge mall, Marina Bay Sands is enough to see from a far however watching an F1 race from the top of the hotel is cool (and is cheaper than doing the view point thing they usually offer).