As above, there are a lot of answers to that question, and it depends on crop/full frame, shooting style, and space available.
If on crop, I'd say something around the 30mm mark, either the sigma 30mm f1.4 if you're on a budget or the 24mm / 35mm L lenses if money is less of an issue.
On full-frame, I guess the 50mm f/1.2 will take some beating, or the 1.4 or the 1.8 depending on funds.
Those should cover the 'walkabout' bit, shooting groups of people though might require something wider depending on how many people are in the group, how they're arranged, and how much space you have to work with.
For street stuff, I'd prefer something longer personally, probably the 85mm f/1.8 on crop and 135mm L on full-frame.
Lots of possibilities, take your pick!
Chris