Clear small plastic lidded containers (the kind you can find at the dollar store are perfect)
Nick-knacks, doo-dads, etc.
Step One: Collect your stuff. Keep in mind that sound is just as important as looks, so a boring wooden bead or some plastic water bottle lids are perfect just because they make so much fun noise. I also told the people I knew what I was doing so I was able to get things like feathers and shells and that little table that holds the lid of your pizza box up... you know, with the three legs?
Here are some of the things we used for Ronan's blocks:
Thread in every color of the rainbow, buttons, foam shapes (sun, clouds, etc), Feathers, shells, fake flowers and leaves, fun shaped sprinkles, puffballs, beads, cut up straws, small rubber bands, ribbon, toothpicks, rice... basically if it was small enough it was going into a block.
Step Two: Prep your blocks. I had to cut off some tabs off the lids because I wanted to make sure there wasn't anything he could grab onto and use his herculean strength to pry open. This step included a lot of help from my son... I would throw the finished ones into the bag, and he would take them out again.
Step three: put your stuff in and hot glue the lids on. For the type of containers I had, I found it easiest to put the hot glue in the fold on the lid. Try to keep the contents of the container from touching the glue before it is dried.
And that's it! Simple, cheap, and fun.