Welcome to the forum, carsgf!
This is a good time of year to ask that question because I remember last year on the Gen Dec forum someone posted a pic of a terrarium decorated for Christmas. It had small white twinkle lights and a garland, I think and some decorations, altho I don't remember exactly what it looked like.
At other times of the year you could use various mosses, small figurines, driftwood, shells, birds nest and eggs, tealight or votive candles in holders, or any other small decorative objects you like. Great place for little yard sale finds like fairies and elves or other small figurines. If you have kids there are lots of other cartoon-y possibilities.
You could do a zen sort of thing with sand or fine gravel and a little rake, a tealight in a holder, and a few interesting stones. Maybe a small Buddha if you are so inclined.
I'd probably change the decor to fit the season by adding hearts for Valentine's Day, bunnies for Easter, you get the idea, but I'm a little crazy about seasonal decor.
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Lucky
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The older I am the less certain I am of many things. Patricia Cornwell
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http://inspiration4u.shutterfly.com/action/ 
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