Having things engulf the characters is an excellent way to make combat interesting. It is a lot of fun to actually put the minifig inside the ooze, and Lego cockpit pieces are perfect for this. The build is really simple: just attach the cockpit to whatever similar-colored bricks you have, and leave enough room in there for a minifig.
Air and water elementals also have attacks that engulf people. The same pieces can be used for oozes and water elementals, but air elementals should be a bit more loose. Instead of making an enclosed space, just pile up windshield pieces:
And of course, no game of D&D would be complete without a Gelatinous Cube:
This is an actual cube; five bricks high is the same distance as six pips wide. It is exactly 10'x10' on the dungeon tiles, the proper size for a Gelatinous Cube, and most minifigs will fit inside.
As you can see from these pictures, you do not need to spend a lot of time building clever or complicated things to be a Lego GM. Just throw together something that conveys the idea of what is happening, and let everyone's imagination handle the rest.