Not sure the time but maybe you can plan like below.
if time is allowed,
Uber from airport to Lavendar -> Uber from Lavendar to Mount Faber cable car terminal -> Cable car to Sentosa
Return: you can take cab/uber from sentosa to lavendar , Or Mono-Rail from Sentosa to vivocity (shopping mall - having dinner) and then take cab/uber/MRT to lavendar.
If not enough time,
take uber from airport to lavendar / uber from lavendar to sentosa. (I expected the pickup point should be clear in the hotel and airport, so should not be a problem).