Cotton Woven Bath Rug - Restoration Hardware.  I swear by these...durable and easy to clean.

RH's Cotton Woven Bath Rug:Our cotton woven bath rugs feature unmatched quality, texture and absorbency.

Recycled Towel Bathmat

Turn Old Towels Into A Soft, Sophisticated Bath Mat

Upcycling a Runnin floor decking into a spa bath mat is a creative IKEA hack.

These 7 IKEA Hacks Will Upgrade Your Entire Apartment

Could definitely do this for the outdoor shower. Spa Bath Mat from IKEA outdoor decking - IKEA Hackers - IKEA Hackers