Maybe it's a big enough area that it can fly to *get in* and it's actually got loads of obnoxious terrain obstacles, pits, etc. These tend to eat up time while being a non-combat thing that increases the tension of getting to the big bad.
Consider having a Dust-Digger like monster that has been increased in size a couple times and while they are going through the cave one of the giant pits is filled with row after row of teeth and long tentacles grasp at them as they pass through. You can just use the Stats for a Roper and the image of it be this massive pit of nastiness.
Maybe since it's his area, you have a puzzle area (and a way to drop hints or bypass it if it's taking too long.) Simple puzzle areas would be runes that may imply types of spells, and they have to cast that type of spell on each rune in the area to open the massive door. Something easy for a dragon to manipulate, but the PCs will probably look through their lists for spells that fit the right thing. (NOTE: I realize a lot of your casters died; so maybe you have wands lying around the cave of different sorts.)
Make it dark so they can't see everything perfectly. It could be labyrinthine and they have to explore around.
Could make it a parent and have a pack of 3 baby dragons as a fight at some point; maybe shortly before the big fight.
Most importantly:
The Big Reveal
You may have an encounter that looks difficult right before the big dragon, you begin the fight but make too much noise! Now the giant dragon rouses and the first fight simply runs away knowing that the real predator is coming. The PCs are now in the real fight and didn't mean to be.
EDIT: Finally, do consider having it roam around a bit so every once in awhile you hear it wandering and get yourself into trouble trying to follow the (echoing) sound. This adds constant tension for when the fight will occur and frustration to the players that will get released when they get to finally fight.
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A mirror maze made of crystal the dragon created by scorching sand with its lighting would be cool.
Have some minor obstacles floating around the lair.
Shocker lizards are not much of a threat at this level but their electric blasts might erode some protection from energy spells if the PCs had cast them. A resetting lightning bolt trap might be fun, too (the dragon uses it to polish its scales in the morning). Either way, something that gives the dragon a chance to detect incoming enemies if the PCs aren't careful. (It might have alarm spells up in a few places, too -- at least some of the dragon's spells should be long-term ones it cast earlier today, since it won't likely get the chance to use many spells in combat.)
Some quicksand traps for PCs on foot (admittedly at this level all PCs should be able to fly), or weakened areas that can collapse.
A princess from a desert tribe who's here to fulfill a bargain with the dragon (the tribe sends someone every couple of years to do scutwork around the dragon's cave, and in return it doesn't eat their herds), possibly friendly with the dragon, possibly eager to escape, but certainly has a high Bluff check. Could be an apprentice of some sort, or a captive; lots of possibilities.
An item that's a plot point for further in the AP.
Some kind of guardian monster that watches the lair while the dragon is absent (could be undead, or an elemental, or something that you think the PCs might not be prepared for), probably hiding in the sand and dust, and only emerges after the PCs pass it. (For some reason many parties are NOT expecting an attack from the rear during combat.) Maybe an invisible stalker? Maybe a golem the dragon salvaged from some ruins somewhere out in the desert?