Yea. I think taking cab, uber is more convenient, especially to zoo and airport transport (with u suitcase etc). To sentosa, u can take cab/uber to mount faber where u can have a nice view to sentosa and h can take cable car there. Or u can go to vivocity(mrt reachable) where is a big shopping mall and u can take monorail to sentosa. Of coz u can take cab/uber to sentosa directly. But I will suggest to try cable car if u have never tried before.
Btw, u can have a walk from Henderson wave to mount fabar if u wanna have some walk. (Of coz better to do on early morning ).