I'm a time traveller, and on my last trip back to the 1300s I befriended several of the locals. One is a fairly wealthy farmer, and the other is the village blacksmith. Now, since they helped me when I was in a tight spot I'd like to pay them back somehow. I know they'd probably appreciate something that would make their lives easier, but the problem is most modern technology is designed to only last a couple of years. What could I give them that would either last through their life/generations or be repairable using local materials?
I'd go with a good pair of heavy military boots.
Anyone who does medieval reenactment with realistic gear will tell you that authentiboots will be either useless on slippery surfaces (e.g. wet grass/muddy slope), or, when fitted with hobnails, slippery on hard surfaces (such as stone paving). On the other hand, a good thick modern rubber sole will grip on pretty much anything. They would be more-or-less reparable with local materials and skills and with some good care (and medieval people knew well how to treat leather) and the occasional fix they'll last a few decades, maybe a lifetime.
Alternatively, if you want to go technology-way, you could bring something like Gravity Light. If you want to splurge, you could have something similar custom-made with better materials and procedures for improved efficiency and life-time.