I've done this twice - most recently was for
Festival of the Arts last month! My #1 piece of advice is to really have a plan for your main goals for the day and keep it realistic. The time will go by so quickly and I like to make sure I accomplish my top priorities! This is my own preference for the day, but I also try to keep it a little less strenuous than park days on some of my longer trips - it helps me keep my energy up and makes recovery a little easier with such a long day (last time I left home at 4:30 am and arrived back home just after midnight). I also try to plan very strategically what I bring since you have to carry everything with you all day - due to the way my plans work out it never makes sense to backtrack to a locker.
My main goal on my last trip was festival-specific events, so I didn't buy multi-pass
lightning lane and hardly did any rides (I did buy single pass for Guardians and got stuck on a way too long single rider line for TT). However I was able to do the Lorcana quests, try all the food booths and special drinks I was interested in, do the painting, visit every artist booth, see the special exhibit, and watch the Broadway concert. I didn't plan any meals since I had so many options from festival booths I wanted to try. I did spend a good amount of time reading all the menus and making a plan ahead of time though, since it's a lot of food to try to fit in a few short hours! On my first trip doing this I managed to do all 4 parks (with at least one ride, snack, and icon photo in each park) in the 9 hours I had.