Shock Cord? It's my understanding that the rubber used by Estes is supposed to break down when exposed to UV light over time. This is supposed to allow rockets that are stuck in trees to come down quicker (and hopefully with less damage) than if elastic cord, Kevlar, or other materials are used. That said, I lost a favorite rocket to a tree (a snag with no leaves to shade the rubber), and it hung there for IIRC, a year. Cows squashed the remains when it did eventually return to the ground (I got back at them for that... Hamburgers, steaks, roasts).
Parachutes? You can
use a mylar balloon to make a parachute with some pzazz. The "string" should be something along the line of Coats & Clark Dual Duty Button and Carpet thread (I know of at least one vendor who uses it for their lines).
photos: luke strawwalker
An additional benefit is that shiny mylar balloons are easy to see in sunny conditions.