Here is a quick 1 page adventure.
Evil Cultist Temple
For characters of levels 3-5
Room 1- Two guards (4th level fighters, chainmail,
longsword and shields, 18 HP each) no treasure.
Room 2- Storage Room. Hidden +1 dagger in a box. Secret door to room 4.
Room 3- Stone Guardian ( 26 HP) no treasure
Room 4- Altar Room. Evil 3rd level Cleric ( splintmail, +1 mace, Potion of Healing, ring of protection from stone guardians, HP 17) Treasure:200 gp.
Room 5- Trap. Phantasmal Force spell. Players see illusionary room ahead but a pit trap is at the end of the hallway ( 3d6 damage).
Room 6- Temple Great Hull. 4 temple guards ( 2nd levels fighters, leather armor, wooden shield, shortsword, 10 HP each) and a 4th level thief ( +2 leather armor, +1 shortsword, potion of invisibility, 14 HP). Secret door to room 9.
Treasure: 250sp,100 gp and 2 gems worth 50 gp each.
Room 7- Library. Pet Rust Monster ( 23 HP) Treasure: spell book ( four 1st level spells, three 2nd level spells and one 3rd level spell) 1 cleric scroll ( two 1st level spells)
Room 8 - Summoning Room. Evil 5th level Magic-User ( Bracers of Defence AC 5, +2 Staff, 14 HP) and 2 Dretchs ( 25 HP each). Treasure: 75 gp, jeweled neckless worth 100 gp, key to chest in room 9.
Room 9- Treasure room. Treasure chest ( locked, trapped with poison needle, save vs. poison or take 2d6 damage). Treasure: 2 potions of extra-healing, wand of magic missiles ( 3rd level- 20 charges), +1 wooden shield, 300sp, 450 gp and 50pp.
Getting players involved:
1- The cult has been killing the local villagers.
2- The temple is a small base of a larger cult, the players might get some clues.
3- Strange monsters have been attacking the nearby village.
4- The cult leader has links to a local baron and a evil plot is in the works.