5mm thick mats are great for knee cushion, recycled mats have nice grip and hold up longer. There are many brands, but it can get expensive so do some additional research before you settle.
Straps for finding space when your arms aren't long enough
Blocks are nice to bring the ground up to your body
A blanket can be anything from a prop to a cover in savasana
Water to hydrate, even if it's not a "hot" class
Almonds or snack to keep an even blood sugar level before or after practice