In addition to the standard 3.5" square white tiles, I like to use the same sized black, tan (Renaissance)and grey tiles, the 4 5/8" diameter Zendala circles, the 10.5" square Opus tiles, etc. Once an artist is comfortable with the tangles (patterns) it is often fun to add color- watercolors, dyes, stains, colored pencils, etc. Also working on other surfaces like scratchboards, clayboards, notebook covers, stationary, etc. is interesting.
Personally, I love working on the natural surfaces of gourds! But the sky is the limit as to what you can draw on- quilts, stones, seashells, sneakers, doors, notecards, furniture, bikes, cars, china, glass, jewelry, and/or whatever sparks their imagination.
When artists combine Zentangle with realistic images, it's referred to as "Zentangle Inspired Art" (ZIA).