While I think it's nice that you want to treat your friend, I'm in the camp not to buy them anything to bring on their trip. I would think flying halfway across the world the last thing they want is to pack anything they may not need. Plugs, chargers, SIM cards and their plans, etc if needed can all be easy purchased there. Even at the airport upon arrival. It's not like they're flying to a third world country.
Offer to do something on this end. Like mentioned, water plants, check for any unexpected mail or packages, turn on off lights, stuff like that.