Just tell them.
Do it in an honest, real way. It can be as simple as, "Hey, thanks! You had a good influence on me." Or, "Man, I would not have made it without you. Thanks."
And mean it. That takes looking them in the eye and speaking from your heart, and they can tell. Special dinner, or treat is optional and fun, but just meaning it, getting their real attention for a moment to say it is what counts the most.
Most people, who are helping others in this way, don't expect much. They do it because it's the right thing to do, and that has it's own rewards.
In my experience as both mentor and having mentors and supporters of various kinds is seeing the other person realize some goal, or see success or just be happy is pretty gratifying.
Know that.
Now, your last paragraph gets right to the meat of the matter, IMHO. Yes, you can't really do enough. This kind of thing isn't about dollars or points, or any other such thing.
It is all about being a good human. When we are good humans, we tend to be surrounded by others who appreciate good humans, and it's catchy too.
So pay it forward. Be a good human when your time comes and feel all that those people you are thankful for do, and then tell them that, and close the loop.
They will very seriously appreciate it, and you will feel damn good about it all.
My .02