My idea of chilling at WDW is - more or less - a bit of commando, but only the parts we find stress-free. We almost always try to go with the flow though when we visit WDW. For me, sleeping until 10am then arriving to face long waits would = less relaxing.
Commando means different things to different people. I am not a fan of getting on a bus 90 minutes before a park opens. I am however a fan of staying at the Beach Club and walking to Epcot just ahead of when the park opens, especially when we stay club level. If we're ten minutes late, so be it.
We especially enjoy the late PM park hours. Early hours are also relaxing for us. If we want to make the day more chill, we leave the parks mid-day or just slow-own/stop rushing/avoid any long waits.
To make the trip more relaxing, consider adding some of the upcharges that make park touring easier, but also don't stress about hitting every show/attraction. If what you are doing is fun, do it. If you find you are rushing too much or stressing about hitting the next thing, then slow down.
The end of May gets a bit warm in the afternoon, so morning and after dark are still going to be your friend.
Getting on GoG as many times as possible would be lots of fun.
In the evening be sure to explore the new area of Epcot that has opened up near Connections eatery. It is a great place to get fun photos at night, and there are tables.
One way to make the trip special is to do special things to prepare for it, perhaps a new haircut and/or getting your nails done up. It might also be a good time to get her a new phone (camera?).
Have dinner at Topolino's Terrace if the menu appeals to you. The view during the fireworks is great, but it does take up an evening.
Do try to get her a pair of graduation Mickey ears if she is graduating this year.
As I mentioned earlier, consider staying club level. You get acess to extra staff, and the lounge which serves breakfast, snacks, beverages, evening appetizers, and 8-9:45pm desserts. Just - if you want to keep trip chill- just don't get too caught up in making sure you are there for every food offering. Go there for breakfast, maybe stop by if you take an afternoon break at the hotel, and every other day go to either appetizers or desserts.