I don’t know if you have DS anywhere in your plans, but there are a lot of options there - both TS and very good QS - and you should have pretty good luck in most places there.
In EP, VN, BG, and Marrakesh would probably be your best options for same day. Spice Road Table would also probably be easy to book. Tutto Gusto is hit or miss; I’ve see it empty and I’ve seen it packed.
DHS will be tough; Sci-Fi ADRs go months in advance and 50s is also usually packed (but you can order off the full menu at the Tune In Lounge, which is attached). Not sure how MM might be.The HBD lounge is another good option; excellent food, but might be crowded. You could also try for the main HBD dining room, but that might be difficult.
In MK, Skippers Canteen is probably the only realistic chance for a same day ADR.
AK will also be difficult. TH would be a long shot. I don’t know how fast Tiffins fills up, but the lounge might be an option. As others have said, Y&Y can be a short-cut with a Landry’s card. Without the card, you might be able to get into the (small) bar where you can order off the full menu (actually, this is a favorite of mine for an extemporaneous lunch, but there are only about 8-10 seats).