Balloons with photos of memories tied to the bottom is a sentimental and thoughtful proposal idea. Let her pop all of the balloons to find the ring. You can fill the rest of the balloons with sweets, glitter or flowers.
Tutorial: DIY Tissue Paper Pom-Poms this tutorial is pretty good. Use square sheets or tissue for a rounder Pom Pom. Tie the middle with ribbon and tied several on ribbon and hung them for a backdrop. Use thin tissue paper and fluff gently.