- Chromecast 2 or Chromecast Ultra's make great gifts.
- Kindle Fire's can be bought in bulk, engraved, and pre-loaded with content. So if you're doing this for a business conference you could for instance pre-load the conference app or some business PDFs on them as well as customize them. Amazon offers bulk purchasing through their business relations team.
- I would go with a portable charger for around $30 USD, which is good for nearly 4 full phone charges. If you want to go above and beyond, then I would also include the Anker PowerLine+ Lightning or Micro USB cable for $10-15 USD. I specifically recommend the PowerLine+ because it's a nice sturdy cable that's also braided.