I usually don't bother training abs separately. But when I do, I usually perform hanging leg raises (or kneeling cable crunches).
Just like almost any body weight ab exercise, the hanging leg raises can easily be done outdoors. But hanging leg raises become quickly more appealing then regular crunches once it's wet outside. You just need to find a soccer goal, a thick branch or a pull-up bar to cling on to.Check out the video below on how to do them. First the regular version is shown, and then a twist which works the obliques quite nicely.
I prefer to do these in a controlled and strict fashion instead of violently jerking my toes to the bar with an overly bended back. It actually feels like a whole body movement where you are in total control of your body. I guess that's why I like them so much.