Day 19 - Favorite Elemental/Plant
Really I suppose this one would be a toss up between the Shambling Mound and something I found on the Wizards website called the Crystal Sunflower.
The shambling mound is just fun. If you don't know it's there then you could be walking around or over it and suddenly get attacked. The surprise factor makes it enjoyable for me.
The Crystal Sunflower is exactly what it sounds like. A sunflower made out of crystal, but it has some minor psionic abilities which make it interesting but random to run across while exploring.