Interested in signing up for some kind of general one-day course for adults in Spanish (but not a language course) where I could just blend in with other students----preferably native Spanish speakers and preferably in Madrid. Any suggestions?

Well, Last summer laduque, MM and I took a course at Cooking Club Cooking School and we all enjoyed it. There are some one day and two day classes I notice on their website. That may be an idea! There is another cooking school, Alambique which is very close to the Royal Palace, which seems very nice too!
Other than those classes, I don't know what else I could suggest. I was looking for some type of culture or cinema class last year, but never found anything either. I would be very interested in whatever you find out. Have fun, whatever you do!!

Yeah. I'm not familiar with any one-day course for adults in Spanish. Most/All are several weeks/months courses, whether intensive or not. The ONLY exception to this might be a wine-tasting course which would only likely last 2 hours or so - and not sure that's what you seek. And beware, most of those wine-tasting courses out there are geared towards tourists so, probably, many of them are taught in English and not in Spanish with Spanish "students". Although the instructor *may* be Spanish.

Saludos, MadridMan