I do like the idea of bomboniere that is either useful or easily eaten! It makes it so much simpler. I love the concept of a small plant, and these can be grown at home for the cost of the pots and potting mix and a few packets of seeds, ore even cuttings! My choice would be for something edible, such as the herbs as shown or a tomato seedling.
This gift would not suit everyone however, and you would also need to consider how they would take the plant home! Another option would be to gift the seeds in lieu of the plant, and such a gift could easily be put into a pocket or purse at the end of the celebration and planted at the guest's convenience.