If I had a time machine, I think one of my hobbies would be to leave unmistakably modern detritus in the past, at sites that I knew were going to become archaeological digs in the present. You know, just to mess with the heads of the archaeologists, and screw up theories and stuff like that.
For instance, I’d go back to Pompeii in A.D. 79, and maybe leave an empty Coke can in Caecillius’s house. Or I’d visit Hastings in A.D. 1077 and scatter a handful of spent rifle cartridges around the former battle field.
Or if I could find out all the places that Time Team was going to visit on their next season, then I’d spend a weekend visiting all those spots in the past, and leave empty beer bottles for them to find in their trenches.
Honestly I think that would be priceless, to see Tony Robinson trying to explain how it seems like the whole medieval period found Miller High Life very popular, and that they appeared to have invented the ‘Bottom Bottle Opener’ several hundred years before the Miller Brewing Company was even formed.