We go to St. Augustine every summer & absolutely love it!
Last year, on our way down to WDW (we drive from TN), we spent one night in St. Augustine, and, then, on our home from WDW, we also spent one night in St. Augustine.
This year, we're spending 3 nights in St. Augustine BEFORE we drive down to WDW so we can have a couple of good beach days.
We love the beaches in St. Augustine, & there are some really wonderful restaurants in St. Augustine too.
I will say, though, that, last year after having been in WDW for a week, coming back to St. Augustine felt a little "flat" - not that there was anything wrong w/ St. Augustine (besides the Bridge of Lions construction), it just couldn't compare to the magic of DW!
Anyway, we've been to St. Augustine numerous times & I know the city almost as well as I know my own. Let me know if you need any info!