Small wedding question re: after ceremony

We are getting married at SF City Hall. We're renting one of the balconies for an hour and are inviting approximately 30 people. We aren't having a reception afterwards, but want to go out to a restaurant in SF. It's meant to be "if you want to come, great!" Very casual celebration. I don't want to reserve rooms as they have food and beverage minimums. This is all due to keeping costs low. Has anyone done or heard of something like this? How did you handle it? Invite wording and such. This is 2nd wedding for both of us so it's taking on the feel of a small wedding mixed with an elopement.