I don't have one yet - my sister has bought me one that has a small packet of different vegetable and flower seeds in each "window" but we haven't been able to meet up yet for her to hand it over.
This reminds me of one year when our children were small and we were very hard up as the mortgage interest rate had rocketed (in the 80s.) I saved their advent calendars for the following year and refilled the little plastic indents behind each window with melted chocolate from a small Cadbury's bar. All went well until Christmas Eve when my oldest DD opened her last window and announced, "Oh look, it's a....BLOB!"