In general there is nothing better than what you said: Go select a hill and start running. If you have (possibly experienced) company all the better. There is one point that you should keep in mind though: Running technique.
On flat ground technique is already important for efficiency and to go easy on the joints. This is even more important when running down. You need to take care to always run on your forefeet and make ground contact with legs slightly bent, otherwise knee and/or hip problems will ensue. This is especially difficult, as one usually starts a run at the lowest point, so in the end there is a descent. Therefore you must always reach the top with energy reserves for the final descent. For me this is very difficult, as a "summit" is always a great motivation, I need to actively break me down. Another option is to run from a high point or to a place where there is some sort of transportation down.