I wish I'd had the opportunity to work in finance/IB for at least a little while. It's something I was always interested in, but would have taken more work to break into (vs. getting great-paying jobs for tech pretty much right out of high school).

Or consulting. I think I'd pick up interesting skills there. I might still follow this path by getting my MBA.

My biggest frustration is that professionally it's very easy to top out in tech. I'm still quite young and already make more than most developers. Really the only way to significantly level up professionally is to take on more risk (by founding a company).