I think if you thought something was absolute you wouldn't have a way to prove it. However, I think there are some things we choose as absolutes, like gravity and the speed of light, or else we would have trouble defining the universe in understandable terms.

In a sense, there is something absolute ... all the stuff in the universe. Try this experiment: if you swing a bucket of water around you on a the end of a string, the bucket pulls outwards. Now in Newton's universe, this is just centrifugal force - the bucket is trying to continue in a straight line, so you have to pull it to make it turn and Newton's third law pulls back. But how do you know you are rotating at all if everything is relative? How do you measure when you've stopped rotating without an absolute standard for non-rotation. In Einstein's universe, there is no such absolute idea of non-rotation. So how come the bucket pulls, then? Do the calculations assuming you are not-rotating, and according to General Relativity, you look out into space, and see all the galaxies whizzing around you. The more distant ones seem to be travelling at almost the speed of light as they race around. This means that relative to us, they have a huge mass, such a huge mass in fact that it pulls the bucket up and away from you! So in Einstein's universe you know you aren't rotating when you are not-rotating with respect to the contents of the universe. The contents of universe form the absolute standard of measure against the universe can exist!