When I was in college and living alone working late nights, my grandfather gave me an old school metal whistle. Man that thing was loud. I never needed it, but it certainly would have attracted attention anywhere.
Really though, I find that being confident and looking people in the eye can go a long way. If you appear sure of yourself and look at strangers like you'll remember every detail about them, they tend to leave you alone. Plus it backs up your instinct as to whether they seem trustworthy or not. Looking at the ground and looking scared may make you a target as would fleeing. Maybe handle everything as if it's a animal. Look big and if it gets too close, bop it in the nose!
As for camping, camp in groups if you're more comfortable. I've camped alone a lot and if I'm in an area that makes me nervous, I usually look for somewhere more hidden and set up when no one's around to see I'm alone. When you're in your tent, no one knows if it's a man or a woman.
I read an article which said if you are at a park to place two chairs around the fire rings so it looks like two people are on the site.
Now, after having camped solo at state parks, I don't practice this. I feel safe without a second chair.