Whether you want to do a light spin or rest is up to you. It varies for different people and whatever else they have going on (e.g., Sunday recovery ride might be easier than getting on the bike on Monday.)

You might find that you are a lot hungrier tomorrow, but if not, then just resume your usual diet.

I often get abdominal cramps after a long ride or a particularly intense ride. I think some of it has to do with the electrolytes and sugar I ingest. Some of it is just the stress of the ride. It tends to be worse when it's hot. From what I hear, it's pretty common.