"Du kannst spazieren gehen oder so."
Sure. For example. "I'm going to be busy for an hour or, so I can't talk to you. You can go for a walk or something." If you wanted to be more complete, you could say ". . . or do something else" or "or do something else of your choosing" "or do something else, just don't bother me now."
The point isn't only that it's not proper, which it isn't, but what meaning does it give. Does it mean "go for a walk or something else (anything - just get away from me)" or what? I see no clear meaning in English (as another American). The meaning in German is similarly unclear to me. At least, thank to duolingo I now know how to translate it, even if I don't know what it means!
In my experience, it's very common to say things like this in English. It's a noncommittal way of letting the complete thought trail off. Basically means "You can go for a walk or something else (other than going for a walk)". It conveys that's one option, but there are others you just aren't bothering to mention. Does that make sense?