I was back in my park (Yellowstone) for 4 full days-3 nights in Gardiner, 2 in Old Faithful. I lived and worked in Yellowstone for most of the aughts so was comfortable with not seeing everything. Also, it's late August so didn't go to Lamar to see animals because they generally aren't there.
Here's what we did.
Arrive-dinner with friends
Day 1-Mammoth VC, northern range boardwalk, tower falls, dunraven, brink of lower falls, Gardiner school playground
Day 2- walk flooded Gardiner road (niche, this road was a daily part of my life, i wanted to see it), walk most of Norris, dinner with friends
Day 3- pack up (takes forever), Artist Paint Pots, Hayden Valley, ice cream at lake with friend
Day 4-OF area. Midway boardwalk, Foutain Paint Pots, Midway overlook, OF visitor center, Geyser Hill ranger walk, dinner in the Inn
Day 5-today we leave. Will see what the fam wants to do. Will probably walk a bit more around OF area. Stop at west thumb. Tonight, soaking at Thermopolis
My kids get very whiny with longer hikes right now which limited some of what we did. Also, they were super excited to see elk and bison, looking for hours through a scope to try to see wolves wasn't on any of our wish list. 4-5 days isn't enough to see all of Yellowstone but neither is months. We'll be back in a few years and hopefully the kids will be up for backpacking and seeing an entirely new park.