You mentioned Groupon. I've gotten good deals through them, but one lesson in Washington, DC. For a visit there, we bought ahead of time through Groupon one of those All You Can Ride tour bus deals.
When we got there, we learned there are actually several other bus companies doing the same thing, and one of them had a different route that could have worked better for us. Those other lines at least seemed to run more frequently too.
But the deal we bought tossed in a harbor tour we liked. Mixed feelings here, so all I really want to say is to check your options before plunging in on the Groupon deals.
Romelle