Though unfortunately I don't have the module anymore, I still remember from "Lost Tomb of Martek" the glass sea and the cloudskates (ok I had to look up the name) that rode upon them. I don't know if it will fit into your campaign (either as an actual encounter or as window dressing) but I think that was a cool idea.
I googled a nice link to a picture of the module, which has the craft on it:
A Druid of the Desert has laid claim to one of the watering holes on the caravan route. The caravan does not have enough water to go around the druid, and its too far to go back.
A few modifications to the druid's spell list can make the battle quite interesting:
Obscuring Mist becomes a strong wind that blows sand everywhere (not a sandstorm)
Entangle can be used with the dried up plants around the oasis.
pass without trace would let the druid harrass the party and escape leaving no tracks in the desert sand.
Sleet Storm can be call sandstorm.
Call Lightning can instead create sinkholes under someone.
The druid could use a pack of desert scavengers (like jackals) as animal companions.
What's the Guard/Secret-Agent-Man up to? Might he put scorpions in the PC's boots during the night?
Do the PCs know he's a spy? What would it do to your story line if the PCs and the Guard/Spy get separated from the caravan during a sandstorm? If they know he's a spy arrange a set-up where they have to cooperate to survive; or they can help him while spying on him. If they don't know he's a spy it might be cool if he saves their life, and they save his - and later he betrays them. That'll get your players into the game.
Consider planting some potential plot seeds that you can draw upon if your main plot goes astray later. For instence you stumble across a stranded adventurer who was magicly transported here to be killed of thirst or hunger, and the caravan happens to run into him and save him. He doesn't reveal much of what happened and later you can have him show up again and tell his story or ask for more help, or even do a favor for the pcs. Stuff like that gives you a fallback for when things go wrong.