When my son was small I'd put cheap bits and pieces like sweets, pens, crayons, puzzle games and maybe a book. Always chocolate coins, an orange and a walnut, the latter two of which he'd just put straight back in the fruit bowl, but they had to be in there. It's tradition!
I'd wrap everything in the very cheap paper you get in pound shops.
Go to somewhere like Wilkinsons. They have lots of ideas for stocking fillers. Depends how old your children are but the ones I like are the little wooden toys and puzzles all costing a pound or two. You don't have to wrap them all, up to you.