There are many great places to take your date out in Sydney. If you want to have a bit of fun together in town, then areas such as Darling Harbour, namely Harbourside, Cockle Bay Wharf and King Street Wharf are your winners.
There are some great waterfront restaurants that will keep your mate happy, such as I'm Angus for a quality steak pie, and any of the Italian restaurants along Darling Harbour, especially Baia The Italian are great restaurants to take your date out to for dinner in particular. After dinner, head over to The Star, and if you fancy a game or two - go for it, otherwise do not leave without buying a couple of macarons from Adriano Zumbo.
If you both are feeling fit, and it isn't too windy, then why not go for a romantic walk towards and along the waterfront (overlooking the Sydney Harbour Bridges and the Anzac Bridge respectively) along Pirrama Park at Pyrmont.
Once you are done, head back to Darling Harbour, and if you are in the mood for it, catch a movie at the nearby Event Cinemas. I know this all sounds simple, but the subtle things make the huge difference, and this entire area makes for a great night out.
You might be lucky to catch some free or inexpensive street entertainment along Darling Harbour on the way back from Pyrmont towards the cinemas.
If lunch is more of your thing, a day out in Manly is also great to take your date out, especially on a weekend in fairly great weather. Sit outside on the Manly ferry if you can as inside the ferry can get stuffy on a humid day.
Otherwise stay at the Quay and eat at any one of the eateries along East Circular Quay, as you are heading towards the Sydney Opera House. If you are both up for a walk, then walk through the botanical gardens and head to Harry's Cafe De Wheels for lunch, and then stay nearby for a waterfront coffee afterwards. Maybe even stop by for 90 minutes of yoga nearby at the Boy Charlton Pool, if their timetable coincides with your schedule.
The drive from Brighton Le Sands to La Perouse is also great fun, especially at around 11ish on a Sunday, and then head to Danny's or Paris for a chicken satay (or fish) burger with chips and eat it with your date at the nearby park across the road.
Any of the restaurants in Balmain, Brighton Le Sands (near or on the beach), Darlinghurst and Leichhardt (in the Italian Forum) are also great places for lunch and/or dinner dates. Enjoy.