For my Nature Journal I have been preserving leaves in glycerine. You dilute one part glycerine to 4 parts water, and stand the stem in it for about two weeks. I have found this is quite successful for gum leaves, and you can soak the whole leaf instead if it has no stem; however the leaf has to be fully green or it doesn't work. Not all leaves take to it - geranium leaves just wilt and die within a few hours. I have used some gum leaves, attached to a machine-made cord and a tassel made of silk thread, as a decoration to my Nature Journal.
If you have been experimenting with papers and/or fabrics, try using your results as a book cover.