I love plants, especially little ones that fit in terrariums.
Here is a terrarium I've had for a couple years now.
One of the funnest things about terrariums are the little surprises you get. Ferns, for instance, are very particular about the conditions they germinate in, but if your terrarium is damp enough, you might get lucky and find a little "Irish valentine" (aka a heart-shaped fern gametophyte). The fern in the terrarium pictured above was a volunteer. It's a happy little fern. I collect moss from outside to cover the soil (this keeps moisture in) and that's like getting a grab bag of little plants. I usually pull out the grass, but I've grown some adorable little shamrocks this way.
Here's an easy step-by-step on how to assemble your own little world.
What you'll need:
1. A deep glass container or fish bowl (make sure it's clean)
2. Potting soil
3. Clean gravel or decorative glass bits (buy here)
4. One or two tiny plants
5. A shovel or spoon
6. moss and decorative rocks or figurines
Step 1. Carefully pour close to an inch of gravel into the glass jar or bowl.
Step 2. Add 1 to 2 inches of potting soil and position your plant(s).
Step 3. Add more dirt to cover the roots of the plant, then water.
Step 4. Arrange moss and any decorative knick-knacks.
Don't forget to water as needed (this depends on the types of plants you have). You might also want to keep a spray bottle handy, if you have ferns, since they prefer high humidity.