The question is about leaving something behind, rather than having for yourself in the past. Most of these suggestions would decay in next to no time.
I think I'd choose something with longevity, that would survive millions of years of weathering -- a pyramid for example, which is modern because it would be constructed with modern machinery.
To communicate anything important with the future of that time, one would need to store information in a format that is capable of surviving millions of years too. Geometry of the pyramid for example, whatever that might say can only be said through ratios of lengths.
Things can be inscribed in rooms hidden within the pyramid, but it may not be immediately obvious that there are any such rooms. If it is left to erosion and decay to expose such rooms, the information might decay before it is learned. So the whole must be carefully designed to encourage investigation at the right time. If the reader is advanced enough to decode the external information, then they're advanced enough to use that to find and comprehend the important information that's on the inside.
If you could go millions of years back in time and leave one modern item behind knowing that one day it would be found (fossilised, or petrfied etc) what item would you choose and what do you think the reaction to its discovery would be?
I think I'd choose something with longevity, that would survive millions of years of weathering -- a pyramid for example, which is modern because it would be constructed with modern machinery.

To communicate anything important with the future of that time, one would need to store information in a format that is capable of surviving millions of years too. Geometry of the pyramid for example, whatever that might say can only be said through ratios of lengths.
Things can be inscribed in rooms hidden within the pyramid, but it may not be immediately obvious that there are any such rooms. If it is left to erosion and decay to expose such rooms, the information might decay before it is learned. So the whole must be carefully designed to encourage investigation at the right time. If the reader is advanced enough to decode the external information, then they're advanced enough to use that to find and comprehend the important information that's on the inside.