If you're looking for something more traditional I'd recommend heading to: The Lunar Markets in Pyrmont for some tasty food http://www.au.timeout.com/sydney/aroundtown/events/42096/lunar-markets. The Hunter Valley for a picnic. Sokyo Sushi bar in Pyrmont has a set menu for V-day for $100 a head. The Museum of Contemporary Art at The Rocks and check out an exhibition http://www.mca.com.au/whatson/. Sydney Boys High School in Surry Hills to buy delicious baked goods at the Flour Market, entry is $2.
Lugarno Seafood. If you are after something a little romantic with exceptional food then I would highly recommend Lugarno Seafood. Romantic soft music and water views not to mention the heavenly desert menu. http://lugarnoseafood2-px.rtrk.com.au/ Phone : 02 9119 1809 or 02 9119 1809