http://www.disboards.com/showthread.php?t=2959331 outlines all the Halloween activities. Probably the biggest ones are that the Haunted Mansion becomes Haunted Mansion Holiday (Nightmare Before Christmas-themed) and Space Mountain becomes Ghost Galaxy (new soundtrack/effects). There are also a lot of decorations, an area with Disney-carved pumpkins, and villains meet-and-greets.
There are several extra-charge Halloween parties (with a small parade and a villains-themed fireworks show, trick-or-treating, etc.). These result in
Disneyland closing to the public at 6 or 7 PM on those nights. It seems likely that eventually World of Color will get a Halloween show as well, but so far that hasn't happened.
If you want to go when ALL rides are running, Halloweentime may not be the time to do it. Much of the season is relatively busy, between the popular Halloween decor and many families being on Fall Break....but last year, Pirates of the Caribbean was down for the whole season! Near the end of the season, It's a Small World invariably goes down for its conversion to Small World Holiday (Christmas theme).
Even in the summer, you can't GUARANTEE no ride refurbs, but July and early August are some of your best bets. Late August, Disney has started doing refurbs sometimes. I've had good luck with late November/early December, but it's not at all unheard of for Disney close rides for refurb then.