Well, I don't know about Dec 21st, it might be too jam-packed to do it, but on a normal day:
watch the E. parade from near Golden Dreams theater. I sat across from Jumpin' Jellyfish, but others have sat closer to GD and gotten out faster.
As soon as the parade ends, start walking out towards Grizzly. Go either way around Grizz; going to the right is a longish path that takes you through a smoking section, and it comes out to the main path just after Grizz. Going left takes you on the main (more busy) path between GCH and Grizz.
Continue on the path, go past Soarin', then out of the park, cross the esplanade, and on a NORMAL day you'll probably be in
Disneyland before the parade even gets to the Sunshine Plaza.
You're not going to get primo viewing, but you'll at least be there.
But that close to xmas, all bets are off, especially in finding any spot to view the fireworks, from what I've read!!!