Nothing really came to me off the top of my head, to be honest.
Everything that I have regretted in my life (both doing and not doing) was something that I could not do if I lived for another hundred years. Things like, not showing enough love to my high school girlfriend, not having enough fun in college, not making more lasting friends during those years.
Now that I have passed those years by, my regrets would be not becoming the better person that I've wanted to become, and that I was never able to find a proper "best" friend. The first one will come with time and self-awareness, and the second will follow from the first.