1: 1 Pixie (Basic Rules) or good-aligned spirit of the troll, now released. Might give some kind of help, treasure or magic item to the party.
2: 1 Willowisp/malevolent spirit, released from troll
3: 1-2 Giant leeches (Expert Rules)
4: Quicksand. First character in marching order makes a dex save; 2nd makes a dex save at advantage to avoid. Roll a d4. This show how quickly they go under. A 3 or 4 means member can be saved by quickly throwing a rope. 1-2 rounds means they go under right away and a more radical and risky rescue must be attempted in order to save the player(s)
5: 4 giant toads (Expert Rules)
6: 2-8 Giant centipedes (Basic Rules)
7: Thick Fog. Party gets lost and walks in circles for a half day, AND, roll d6 again for another chance to have a wandering encounter.
8: Insect Swarm (Basic Rules)
9: Sourtongue’s lair is discovered. Determine potential treasure using Expert Rules.
10: 5-8 Lizard Men (Basic Rules)
11: 3-6 Robber Flies (Basic Rules)
12: A dangerous or carnivorous plant, like the Mantrap (2nd ed)
*italicized encounters can only happen once