Aseos is the usual word for toilet. I have even seen it on direction signs in Madrid. If you need one, go into any cafe or bar and buy a cheap drink for €1-2.
I've only visited in February and March. Salmorejo was always served cold and I tried to eat it at least once a day. You can also buy it in supermarkets in 1-liter cartons (like OJ), but then you have to eat (drink really) it without the chopped egg and ham garnish. Still yummy.
Potato chips. A common snack, seems like all are fried in olive oil. Ice cubes are ubiquitous, yes even in winter.
I've had good sangria in Cordoba, Granada and Barcelona. It's fine if you want something icy cold and alcoholic and don't care for beer. I probably shouldn't, but I always eat all the fruit too. Enric, I promise when we are together not order sangria!
Allow time at the train station. Madrid's is very large and some find it confusing. For fast trains you need to go through a quick security check, though there can be a line in Madrid. Allow time for that and for finding your platform. Train doors close 2 minutes before departure. Be on the train before that!
Most of the souvenirs are well-priced and good quality, fun to look at and take photos, great for gifts. Some people do their Xmas shopping on their trips. Hand-painted wooden fans are a great gift if you live in a hot climate. If you are buying several items, bargain for a discount.