Good question. I'm going for a 5 day trip in April with DD8 and DS4. We're staying in POFQ and I'd like to enjoy the resort a bit so I think we're only going to do 2 park days, but it's so hard picking which ones to do! Where are you staying? I find that it's so nice to take time away from the parks, even though it's tempting to want to cram everything in.
We're definitely going to MK for Day 1, and we'll either rope drop and call it an early night, or take our time in the morning and stay till closing. Either way I'll pick our FPs for after noon so we can have some flexibility.
I'm really partial to EPCOT because it is so open and never feels crowded to me. The kids can explore a lot and we are not big "ride people" anyway. But, with all the construction, I may just have to skip it this time. I find DHS to be the least "relaxing" park, but it may be our best choice because of Toy Story land for my kids. Again, we won't do a full park day here either. We max out around the 8-10 hour mark.
I think the best way to have a relaxing trip is to plan for plenty of down time. We like to hang out by the pool, walk around the resort, ride the monorail, and leave time for spontaneous moments. I don't find the parks particularly relaxing, so to speak, but attitude is definitely everything. As long as you're not rushing from ride to ride and don't feel like you need to get a certain number of things "done", you're bound to have a lovely time wherever you go.