Just in general, taking a nice (long) walk always calms me down. I can hardly think of any times it failed me Walks also refresh me and give me inspiration when I need. Walks through nature, preferbly of course And it doesn't matter if its pouring down, because I actually find that refreshing

As for nightmares.. it has been a long while since I can remember having a real nightmare that left me really anxious and tense when I woke up. I tend to snap out of it pretty quickly. If not, I think I would just sit up and let myself calm down. What might help is thinking about your dream and imagining a different, happier ending I think there is actually a technique like that.. can't really remember though.