Cover your alternator, exposed harasses, battery, etc. Aluminum foil is nice since it contours to the shape, but just don't be dumb and short your battery terminals.
I'd pick up some P21S engine cleaner. Spray on a cool, dry engine bay. Let it soak, scrub up with a brush. Rinse off. Best stuff I've used. Towel dry. Then dress up the vinyl, plastic and rubber, with some 303 Aerospace Protectant.
Engine bay will look like it just rolled off the show room floor.
"We live for a good time, not a long time."
RIP Colin McRae