E.A.:

I would assume you're planning on protecting the electrical/computerized components under the hood. Plastic bags work well; Saran wrap also works if a bag will not fit easily over the component.
As far as products are concerned: you note your engine compartment is already clean. The easiest product I've used over the years is Dawn dishwashing liquid, mixed about 1:4 with warm water. Apply it to a warm (not hot) engine with a paintbrush. The cheap-o nylon brushes work best because they're a little stiffer than natural bristle.
Let it sit for about 10-15 minutes, spray off with a 'gentle' spray from the garden hose. This is not the time to use your force nozzle! If the engine is warm enough, it will dry in about half an hour.
Work in an area where you can control the runoff; the detergent itself is reasonably 'harmless' but the oil and grease you remove may not be.
Final wipedown with a damp cloth eliminates water spots; lacquer thinner will restore the dull finish to hoses and belts...only the outside of the belts, please.
Once it's clean, this routine once a month or so should keep it sparkling.