may i ask why you're leaning towards sentosa?
Most of the hotels there are in earshot of dining and music spots - so its not really an RnR vibe. I have some mates who live on the resi side of sentosa and that's a little nicer... there are quiet waterfront walks you can go on and admire the houses.. but the whole thing feels a bit truman-show-esque.

I would agree with ESpen. Stay in the city somewhere.. I would suggest Robertson Quay if you want to be a little more where local expats go. Lots of nice restaurants and it's on the water.. to your left is Clarke Quay if you want a more touristy-drinking area too.
It's in MRT/bus distance of anywhere and there are so many spots you can walk around.

I would have a look at Airbnb too - some of the ones in the abovementioned area that are modern and that I know have nice pools, tennis courts and gyms are; Martin Place Residences, RiverGate, Inspira.. you get the full resort feel minus a buffet breakfast (which you don't need because you're in Singapore).

Let me know if you need any more advice.