Time you can visit. all you can take is pictures, all you can leave is footprints. -- Anon Guest
There are rules to time travel. Some are invented on the fly, others make more sense than that. First and foremost is no excessive interaction. One cannot, for example, go back in time to tell Freddie Mercury or David Bowie how influential they become. Especially towards the start of their careers. One cannot also travel back in time and leave an iPhone in Steve Jobs' office.
One cannot also go back and take things unless they are famous for going missing. Even then, a duplicate must be fabricated and left in its place. Filled with tracing agents in case the object is ever found again.
In time travel, information is the most valuable treasure. What happened to the relics of the past is more important than the relics themselves. Of course, recovering them is a bonus, but knowing is more than half the battle against the sum tides of ignorance and spite.