I've lived near or in St. Louis all my life, but decided to give a trolley tour a go one day. Glad I did! Dusty, the driver, knew a lot about the city, and he's a funny guy. I was on with some folks from Cleveland in town for a wedding. Dusty was able to provide them info on the city, attractions, restaurants, etc.
It was fun, and I admit to some pride, listening to the city's guests show genuine surprise (and even awe) at what the city has to offer, much of which Dusty could explain was "absolutely free." Ironically, one of the trolley-riders was wearing a Route 66 shirt. When Dusty told our group we had literally just turned on to a stretch of the famous road, the passenger exclaimed "No way!" and eagerly scanned the area.
Of course, as a native, I knew about Route 66, most of the restaurants and which attractions in the city are free. (Here's a hint - a lot of the big ones in town are, excluding the ride to the top of The Arch, but including the museum below it.) Here's the thing - I learned a number of things about the city I never knew, and was reminded of others. I won't give spoilers, but there were several things I learned ranging from relevant notes of the city's history, to interesting facts/trivia about local businesses to driving by the childhood home of a favorite local celebrity.
If you're visiting St. Louis, take the tour. It goes quickly - just short of an hour and a half, and it feels like less than that because it flies. But you'll learn a lot and can use the tour to help set your vacation itinerary for more in-depth exploration.
If you're from St. Louis, take an afternoon to re-visit your city. Combine this with a day at the zoo or a brewery tour (yeah, those would be a couple of the freebies in town), and you'll have a great day.