Egg cartons....... easy...................
I am completely sold on using the Jiffy seed starting trays and plugs. Never tried them until a few years ago. So much less mess, and great results.
They're re-usable; just buy new plugs the following season and wash out the trays well before re-using. They come in many sizes. I like the 16's for peppers and tomatoes and I use the smaller plugs to do entire flats of whatever annual flowers I need for planting. (Zinnia, Marigold, Calendula, Sunflowers) Love, love, LOVE them!