Yep, I agreed with that harness thing. Looks like my cat feels uncomfortable and she just stays there and like you said Sootyca, giant flat slug with fur lol.
Sometimes cat feels okay with the harness, maybe need some kind of training session first, my previous experience, the indoor cat seems not so like with the harness, maybe the majority time of its life spent inside a home, it feels weird and uncomfortable.
My personal opinion, the best thing to entertain outdoor cat is to bring him outside and put him inside something, I mean the cat backpack, as far as I concern there is a backpack that has something like a space capsule, your cat can enjoy the beautiful scenery as well, I do a quick search,
see here. Lol, I wish I could go hiking with my cat.