It's just wonderful and that is all. Straight to the middle of the bridge and back is equally fun and takes less time. It's nice for the less athletic. Biking is a good choice, too, but takes more planning. It sometimes requires the ability to stop pretty fast for slow moving groups.
The trick is, and I learned this the hard way, get there before the sun goes down. After that, it's really creepy trying to get to the start of the bridge - under an overpass where it's dark and smoky, through the tagged chainlink tunnel of doom, ending with a locked gate. Not romantic.
My fantastic advice is this: go before they lock the gate.
To catch the sunset, start your walk about a half hour before and dress warm, as it's almost always windy on the bridge and cooler than people expect. The parking lot is pretty nice, even after dark - unless I was lucky last time, it's even free to park there. If your date's into history, there's a statuary tribute to maritime heroes touching the parking lot on the opposite side.
Chinatown is a must-see for visitors, but the guided tours are generally great citywide. There are perks to the Chinatown tour. The guide may stop by a fortune cookie factory - it's happened before, and ... yum.
I like to take BART to get there. The best is to bring your bike or rollerblades and skate along with so many others. Sunday afternoon is my favourite, because there really aren't a lot of people there.
Note: the cafes that are open during the week are usually closed on the weekend. Get your Starbucks ahead of time.
Option five - Drive down Lombard Street - the 'twistiest' road in the city! Don't eat first.