A bagatelle is a trifle or a trinket — a little thing without too much importance or value. You might wake on Christmas morning to find your stocking full of bagatelles.
Use the noun bagatelle when you're talking about something that's purely decorative, like a bauble or knickknack, or that's very easily done. For example, you might say, "Oh watching your cat was such a bagatelle — I really didn't have to do a thing!" The word is French, meaning "knick-knack, bauble, or trinket," from the diminutive form of the Latin baca, or "berry."