A deep gorge with a stream at the bottom, just too wide to jump across. The site of a previous battle; Who was fighting? Who won? Is it important? A ring of trees growing in a perfect circle. A huge, spherical rock.
I intersperse unusual but sometimes meaningless things throughout my adventures to give my players that 'part of a world' feeling where not everything revolves around them or happens because they take some action.
Interesting and unlikely geological and natural structures can make for a few minutes interesting role-play as the characters try to figure out if the ring of trees is a clue, a lair, a portal, or just some trees....