you can't surf in singapore but you can consider windsurfing perhaps. over at east coast sea sports club. just take up the basic certification and rent or buy some second hand gear. been going since 2004 but i haven't gone down for awhile. (vain reasons like freckles).
or others like cable skiing, etc. batam has it cheap and calm, easier for beginners compared to the east coast cable ski ring.
www.cable-ski.com i usually spend the whole day there since there's ferry commute. quite a few teens too. consider making it a bonding session with your tweens/teens. haven't been down for awhile though.
nice one about the malaysia trip. could consider that for my next weekend trip.
to touch more water, you could also take a weekend trip to pulau redang, etc.
have fun!
(i realise my post seemed rather off topic but for breadth.)