Starting in North Beach and running through Cow Hollow, Union Street serves as a Main Street of sorts for Cow Hollow. There are lots of great shops, restaurants, cafes and bars.
Union Street is close to the Marina, the Presidio and Chestnut Street; making it a good starting or ending point to your day or evening.
The main cluster of businesses area runs from Gough to Fillmore.
It's also mostly flat from Van Ness to Divisadero making it a great walking street to take the kids.
Union Street is in one of the nicer areas of San Francisco, so most of the shops tend to be a little more upscale, but there's nothing wrong with a little splurge here or there.
Metered parking does open up so if you're patient, you'll get a spot. But at times, parking can be a royal pain... so taking public transit might be a good choice... the 41 and 45 run down Union.