My first draft dungeon (and I'll be changing it as ideas come in until game time tomorrow!) sees the following encounters in the dungeon:
1. three kobolds arguing over dice.
2. a gate trap. Easy to disarm. Spikes deal damage
3. Gas Spore over a treasure chest. If it explodes, most of the nice stuff in the chest (including trinkets, the best treasure!) is destroyed.
4. Pool of green slime. There's a cage over it, with a leather cord that can lower the cage into the pool. There's a dead kobold inside.
5. A weird altar with candles and skulls. Inside the skull is a gem (natch). extinguish the candles and the god of undeath gets angry and you have disadvantage on all saves versus undead attacks for 1 day.
6. 4 kobolds and 2 trained giant rats. The "big fight".
7. A sword in a stone. Pull it from the stone, and it animates and attacks!
8. A cave in. Lots of hints that it's coming.
9. Kobold hiding behind crates. She has a crossbow. She doesn't really want to fight.
10. A ghoul, chained to a wall.
11. 3 kobolds with 2 skeleton pets, guarding a warhorse (the reason the PCs have been sent into the dungeon).
It's a simple, one hour to create dungeon, but there's enough choices that the players should have fun. Any ways to make it awesome(r) are greatly appreciated. Would prefer to keep it simple.
Three balors riding a terrasque. GO BIG OR GO HOME.
Goblins are not quite the trainers the used to be. You really can't use them in a good amount vs a first level group. That Dex adds to damage as a default means their hits are much more solid than the old days. Then add in that they are designed to be playing Whack a Mole with the hiding rules, followed up with attacking at advantage from being hidden means they can easily overpower a typical group.Goblins are my normal choice - evil, small, smelly and squeal loudly when you hit them. Since the advent of D&D, goblins have been fantastic trainers for level 1 PCs to take on the world.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.