I have seen some nice (expensive) dog bone shaped low surface creepers advertised and considered buying one...
But at a stop at Lowe's I happened to see dense foam interlocking floor mats for exercising, kids play, etc. for under $20. I picked up a set of 4 (1 pack) and gave them a try on my garage floor by changing the
oil and filter on my car.
Wow 
The next day I picked up another set! I use them all the time now. In fact I put my creeper in the back of the storage shed and have not pulled it out in two years and probably never will again.
The mats are much better for me to work under the car with than a creeper as I am on the floor and I find it much easier to slide in and out than to have the wheels of the creeper getting hung up on tools or other stuff. I hook as many of the mats together as I want depending on the job so that they go as far under the car as needed and still have at least one mat clear of the car. Sometimes I run them all the way across so I can enter and/or exit from either side of the car.
When I get
oil on them, it washes right off although I do have some staining that doesn't bother me. They come in various colors but I only bought gray ones for the garage.
I use them outside when working on (under) the RV or on a car that won't fit in the garage. I use them when working under the dash by stuffing it on the floor over the door sill that used to cut into my side or back. I put them on the fender when working on the engine and protect the paint as well as making it more comfy for my old bones. I use them inside kitchen cabinets when plumbing requires attention. I use them when working on my knees in the yard, on the roof, or anywhere I can.
I did miss the adjustable headrest of my creeper but I learned to roll one of the mats up and secure it with a bungee or tape and put it under my head.
I keep at least 4 handy at all times for everything, padding potential hard points like where the car door may hit something hard if opened too much.
I suppose they could even be used for doing exercises... but I have carpet in the house for that purpose
As you can tell, I wish I had invented the things!
I know, you asked specifically about creepers but this is what I found is the best creeper for me...