My first choice for the situation you describe would be a handgun (Glock 26 or S&W 5 shot 2" .38). Second choice would be this:

http://www.taser.com/products/consumers/Pages/C2.aspx

I've seen pepper spray not stop the attacker too many times. If I was going to use pepper spray the Fox fogger seems to be the best.

Next choice would be an expandable baton, although, a female in her 70's might not be able to weld one effectively.

While the knife would be an effective defensive weapon for you or me I serious reservations about it being so for a 70ish woman.