Definitely a ledge. Probably a short one, so anything under a foot tall will work. The coping should be square and as smooth as possible. The wider the coping, the safer it is as well. The reason being that if you go a little too sideways with not-very-wide coping, you're deck will be sliding on the wood WHICH DOESN'T WORK. You can also prevent this by controlling how "crooked" you get, but that takes some getting used to. Wax it down good as well, cause if you catch an edge on an overcrook you'll go over the bars and eat shit. Learn fs overcrooks first, they're easy and you can watch your deck lock into the coping. To learn them, go REALLY slow, or just stand on your scooter motionless next to the ledge and try just locking into the overcrook. Don't swing the deck of the scooter out too much, you'll end up doing noseblunts which a little more tricky. Keep hoping and locking into the grind, going a little bit faster each time. You'll realize that once you start going fast at the ledge, you'll have to lean farther forward than you may initially think. Otherwise, you're back wheel will immediately drop down into a smith. However, if you lean TOO forward, you'll end up sticking to the ledge and getting shot forward, so try to avoid that as well. Patience is key, no matter how much I do this trick, it is ALWAYS frustrating. Think of it like a nosemanual, but your wheel is constantly getting softer and harder and your balance point is constantly changing. That would throw off your nosemanual everytime and you'd essentially have to relearn them evertime you do it. An overcrook is similar in that way since all surfaces grind differently, and sometimes you'll be doing it on concrete, marble, coping, rails, etc. Thusly, overcrooks are pretty inconsistent and require a lot of patience. Holding them across an entire ledge is another battle, similar to holding a long nosemanual, but there are few tricks that feel as good and are as rewarding.
DO THEM.