Styrofoam, like the stuff from take away containers or from the grocery store, can be used to make your own stamps. (This site is in French. I tracked the photo to http://www.funnycrafts.us/printmaking-ideas-for-kids/, but there are no further instructions there. I guess they figured the pictures said it all.)
Use a sponge to soak up concrete mix in whatever shape you desire and let it harden into a hardened version of what you just cut out! Pavers or garden accents? (I would wear rubber gloves while dipping/soaking!) Worth a try.
Beautiful. Oak leaves last a very long time and would work wonderfully for this. I wonder if leaves preserved in glycerine would too. Maybe wax the leaves after painting to preserve them and keep them from shattering once dry.