The best thing you can do is make your primary team-wide interactions remote friendly. In fact, it's really the only thing you can do to make remote work well!

The best thing you can do is make your primary team-wide interactions remote friendly. In fact, it's really the only thing you can do to make remote work well! No matter where the employees are, even if they're all in the same office, you should make decisions on Slack or Hangouts or Github or Jira or whatever. Not just that, but you should have fun in those same places, that should be your water cooler for casual social interaction. I'll assume that you're doing that already, but if you aren't, no event(s) will make up for it. One company that I worked for had an annual party, and many of the remote employees made an effort to visit one of the two offices. The two offices then established a live-stream back and forth during the party. Remote people that didn't attend in person could stop by the streams, but that was probably only fun for a few moments. Offering (not requiring) a largely socially driven (instead of work driven) trip once or more a year is probably a good idea, especially if you have some sort of annual event to center it around like a holiday party.