From the extra info you've posted, it sounds like you've gotten all expectations correctly outlined. Kudos to you! And your SIL and her DBF sound like great people. It takes a village and you sound like you are part of a great one! Offering to pay for everything was a good start and because of her response, I totally understand why you are letting them pay for their own food. My parents won't hear of us buying their tickets (their AP is seasonal and doesn't cover Xmas). Our tradition is an Xmas trip, so they say it is their Xmas gift to themselves, to him AND to us. I take care of my son's ticket and try to buy them food while there, when they accept or if I can have my card out before them

. I do pay for all of our accommodations as my thanks to them.
As far as what my husband and I do - anything we want! Ha! My son loves MK, but we are bigger fans of the other parks. So usually we start off with them in MK and then hop to another park, ride a few rides, drink (around the world if in Epcot) and enjoy an adult's only dinner. We never get wasted since neither of us like that and we meet the family up again in the evening. Wash, rinse, repeat. As we live across the country from my parents, we usually have a LOT of date nights to catch up on, and we try to make the most of it.
My son is on the taller side (no clue where he got that from as his parents are shorties) and as he is getting older (he's in kindergarten) he does more and more with us since he's now tall enough to go one all rides. So yea, usually the morning with the grandparents and then the evenings to ourselves.
Are you looking for specific couples only stuff to enjoy? We mostly do dinners and shows together since we know the kiddo would likely not sit through it. My favorite restaurant on property is Le Cellier. Others will disagree with me, but I love the cozy, dim atmosphere - just seems romantic. Plus I LOVE their filet. I am forcing myself to try some other places this Xmas trip because I need to see for myself, but we've made so many fond memories here, that it will always hold a special place in my heart.