For my three year-old's classroom Valentine cards, I bought a huge stack of Valentine-themed stickers and some fairly plain pink note cards with envelopes. She loved dressing the cards up for the holiday, and I addressed them to her preschool classmates.
It was a fun activity for her, she was able to participate since she can't actually write yet, and I didn't feel like I had sent the valentines to the kids, you know?
Bonus – the cards are character-free and turned out to be cheaper than many of the pre-printed Valentine card sets.
What a great reminder to keep classroom valentines SIMPLE. My daughter is using a neon-colored index card, folded in half, as the base for her cards (bonus: no envelope stuffing). She adds a few flourishes and stickers (which I picked up for $1 per pack of 50!), signs, and she's done. Cheap, fast, easy, portable, and, most important of all, her own independent project.
More, more, more: Valentine hacks!