During the war, shoes (and leather) were rationed in order to provide footwear for troops. Americans were urged to use it up, wear it out, make do, and do without, even for badly needed new shoes.
"Holiday for Shoestrings" is a pun on the popular instrumental "Holiday for Strings".
The boot marked "4 F" refers to the military term meaning that a registrant is not acceptable for military service. To be eligible for Class 4-F, a registrant must have been found not qualified for service in the Armed Forces under the established physical, mental, or moral standards. In this case, the physical limitation is fallen arches or "flat feet," which the elf corrects, allowing the boot to be reclassified as "1-A" (available for military service).
The use of a waffle iron on a shoe here predates Nike's "Waffle" running shoe by 25 years. The first waffle runner, whose sole was made with a waffle iron (yes, really!) appeared at the 1972 Olympic trials.