I wish I’d thought of that. Operation Starbucks
My team has done similar user testing in Starbucks quite a few times.  Here's my $0.02: If you're willing to be bold, walk right up to people and ask, you don't need to offer free coffee or gift cards. We've found this is a better strategy for a few reasons:  1. It's cheaper.  2. It does not dramatically change the number of test users you can get in a given afternoon. And 3. You're more likely to get critical negative feedback if you dont put the users in the position of owing you for the coffee.