Maybe Jethro Tull is there with the speaker, maybe not. In fact, we don't even know if there's a park bench or a park for that matter. But there are some trees! And there are people looking at the trees too, but maybe just one person. We can't be too sure. But those trees sure do stir up some big ideas.
So we can assume we're in a natural sort of setting with trees and all sorts of neat ideas. But the real setting is in the speaker's mind where he starts to contemplate his relationship and the not so simple ways of the world—both as it is and as we might try (in vain) to force it into our own words. It's also a place where you can think for yourself and come up with your own ideas based on the poem's words. All in all, it's a pretty chill spot to soak it all in.