Two things I wish I'd done in undergrad:
1. I wish I'd shadowed the careers I was interested in. I chose a career without ever observing it, and in truth it was a bad fit for me. I didn't discover it until undergrad was over. Now I'm into PT and love it. I could have discovered that earlier and saved a few years of time if I'd shadowed my first career choice and discovered I didn't like it before setting out for the degree.
2. I wish I'd researched and targeted specific PT schools before leaving undergrad. There are some schools that would have been awesome to apply to, but I couldn't because I didn't have their school-specific prereqs. I had the basics - two semesters of chem, physics, A&P, etc. However, there were some schools that required oddballs. One (the only PT program in my home state actually) wanted Medical Terminology. Another wanted an advanced English class. Still another wanted Kinesiology. If I'd researched the schools BEFORE the application process began, decided which ones I specifically wanted to apply to, and double-checked their prereqs, I could have taken one or two of those oddball classes and opened up my options a little better. Some of those programs that have the oddball prereqs are both good programs and low-cost. Getting into them can be of huge benefit financially in the long run, but you can't do that if you don't have their prereqs.