Take a cheap boat ride to pulau ubin, rent a bike and spend the day exploring. It looks like old singapore when everything was Kampong style.
Rent a bike in east coast park and cycle to Gardens by the Bay and back. Depending on your speed this can take about 2 hours.
McRitchie reservoir is a nice place to walk about and if i'm not wrong, there's a trail leading all the way to Bukit Timah Reserve.