Engine Bay - light power wash, water only, to get rid of the grit and stuff just on the surface. While it's wet, about a quart of Simple Green on everything except the K&N, let it sit for 15 minutes. Rinse it all off, start it up and drive for 30 minutes to dry it out. Pop the hood and look again, see what stubborn spots are there. Wet it all down, more Simple Green and maybe a little brush action and a good rinsing.
If you've had oil leaks or lots of grease that's now an abrasive paste for years.....I'd get some shop towels and contact cleaner, get that crap off first.
It's not hard to do, you just have to take your time and do it multiple times to get it perfectly clean. Once you're there, just a rinse once every 6 months seems to keep it nice. Dress the black stuff or wax the painted stuff as you see fit.....provided you still have the patience to mess around under there.