First of all - take a deep breath. You still have some time. Assess the situation. Let's say the players are currently on their way to Hammerfast in the Nentir Vale. You just need to knock together a quick 'side trek' to occupy them for the night. A three encounter delve should just do the trick. Now that we have that decided - where should you start?
For the purpose of this little exercise, I am going to assume you have access to the following:
Dungeon Master's Kit
Dungeon Tiles Master Set: The Dungeon
You think that a dank, creepy tomb might be a nice change of pace so you dig through the box of dungeon tiles and flip through a few until you have this:
Now you need some critters to drop in. Undead are an obvious choice but how about something maybe a little more unexpected too. After a quick search of the Monster Vault you come up with this encounter.
Not So Empty Tomb
Encounter Level 5 (1,000 xp)
2 gargoyle rakes
This musty tomb contains six stone sarcophagi. Two appear to have been opened and two still sealed. Two more elaborate sarcophagi have carvings of warriors on them. The floor is wet and slimy, and water drips from the vaulted ceiling.
The gargoyles wait until the characters enter the characters enter the tomb and then swoop down from alcoves to attack. The wraiths wait invisibly by the two sarcophagi with the warrior carvings on them and attempt to surprise anyone that approaches them.
One of the wraiths' sarcophagi contains a skeletal corpse that is wearing various gold and bejewelled funeral goods worth a total of 1200 gp.
Features of this Area
Slick Floor: Anyone running on the slimy wet floor must make an Athletics check to avoid falling prone.