I've known stickers are a great fine motor activity, not to mention beloved by all toddlers, for a long time. But I've also always known that once they are stuck, children tend to get frustrated and want to pick them up again.
Only recently was I given the idea of sticking them onto wax paper. Voila, non-permanent sticking.
I pick up sticker books whenever I see them for a good price. I've found them at the dollar store and at Joanne's for a hundred stickers for a dollar.