The Mecha suggestion is good in terms of keeping the casual "tone" but you'll have to slurp noodles in front of someone you don't know that well. I'd say Flinder's Lane on Summer Street has a similar vibe but more varied menu items (burgers, salad, pasta). The drinks are good too, but you can always leave and walk somewhere else. Depending on how things go you could walk through Mill River Park before choosing how to continue the evening.

As always, Third Place has a very casual vibe. They have a food truck rather than real dinner items, but it's a safe bet imo.