Aside from extra cables/adapters (TechMatte makes decent microUSB to USB-C adapters--be sure to get the 2nd gen ones), I highly recommend a case and screen protector.
I like cases from Diztronic, which cost $10-12 and do a GREAT job of protecting your device.
I know many people prefer their devices "naked," but I prefer my money in my pocket and not spent on repairs for the inevitable phone drop. If your phone survives a drop naked, luck is on your side and you should count your blessings. If not, expect to pony up some $$$ for some phone doctoring.
I also like the cheap and easy screen protectors from Mr. Shield. They are easy to apply, durable, and cheap to replace (especially since they come in a 3pack).