Everyone has opinions but I thought it would be helpful to get really valuable First Time Trip Tips in One thread, and maybe call it that. What is the one tip that you got as a first timer that you are so glad someone gave you? It can be anything, from planning meals, to timing, to resting, to can't misses, etc.
Mine is: DON'T overplan! There is no way you can do it all. Pick two major events if you're scheduling "extra curriculars" (like tours, or special dining events) and stick to that. You dilute the experience to do more, and run yourself ragged trying to get to everything.
What are you glad that someone told you...and you listened?