Well exercised lungs and a cell phone are about all I'd suggest. If you're not *positive* you can keep someone a foot taller and 100lbs heavier from taking a weapon away from you, then any weapon you carry can be used against you. Most women (like me) are small enough that a man can easily get an unfamiliar weapon away from us. It takes a lot of training and practice to be sure you can hold on to a weapon and use it appropriately in a self defense situation. If you're not willing to put in the time, do not carry a weapon... because then you're just helping your attacker.

Traveling in a group is a good idea. It's not always practical tho. If I'm out late (for me, which is not late for most people), I make sure people know when to expect me, and what route I might take. I keep track of how long common trips take me too... if I'm an hour late from a 10 mile ride, my partner and friends would be *very* perturbed.