Considering where I am right now, how should I be preparing for my upcoming doctoral program?

I was recently accepted into an I/O PhD program on the east coast, set to begin in the fall. Right now, I've been communicating with some of the current first-year and second-year students to glean some info on the program. Things are looking good, and I know I'll be faced with an overwhelming amount of work once the first semester is underway. What I'm looking to do is sort of soften the initial blow.

I took a year off after undergrad to work full time in positions that didn't make reasonable use of my degree. Truth be told, I was originally placed on the waitlist when I applied to the PhD program, but one of the accepted applicants bowed out and I got dibs. I essentially have no research experience worth reporting. My merits include an above-average GPA and GRE scores, what I assume were glowing recommendations, and an Honors thesis from my senior year that won a minor award. I feel I'm going to appear woefully inferior to my fellows, especially if they have a lot under their belts. With that said, I want to prepare myself intellectually and shake off the rust this summer. I'm going to read over my thesis and attempt to draw some additional research topics of interest from it. I'm going to read a book that will expand my vocabulary. I'm going to peruse my old I/O and Statistics textbooks from undergrad.

Any additional advice would be appreciated, especially from current PhD students or graduates.