definitely polo wraps. fairly easy to make, and if you offer unique patterns you can make lots of money!
U could make a lot of money out of browbands
Also you could make treats
You can decorate crops, polo wraps, lead ropes or paint the wooden part of brushes. Tie dying pads, polo wraps, lead ropes and halters are easy and pretty :)
Para cord reins, tie dye pads, polo wraps
Fly veils, halters, bridle charms