A knife is lethal force, not less than lethal.
The first and best thing you can carry is your brain. I'm not being flippant here: your best less than lethal option is to get training in how to use your hands and the world around you to defend yourself. Viewing websites, reading books and magazines, watching videos -- all okay, but far inferior to getting some hands-on work with qualified people. It doesn't take years of martial arts training in order to get just a few simple techniques you can use in a variety of situations. Add to that a determination to stay alert and aware (and again, a little training in what to look for and some ideas for how to avoid trouble or de-escalate it if it starts) and you'll be in good shape.
Pepper spray can be an excellent tool, but too many people carry it without training in what it can do and how to use it most effectively. It's not a panacea, it doesn't work on every attacker, and it doesn't always incapacitate quickly. In addition to all that, people who don't know how their own personal bodies respond to the stuff probably should not carry it, since it's pretty much a given that you'll be exposed to it too when you use it. Can you fight through its effects? How do you know? Again, training makes a big difference here.
On airplanes and in other very non-permissive environments, you have lots of options if you know what to look for. If you have just a little training in short-stick techniques, for example, you can use a simple ballpoint pen to great effect. Without that training, a pen is just a pen.
There really isn't a gadget you can carry that takes the place of training. But with just a little knowledge, a lot of ordinary items can be used to defend yourself.
pax