- Use Slack for water cooler talk, but not just "on brand" links like articles about software. Make or join a #videogames, #movies, #music, and/or #photography channel and talk with people outside your team. If it makes sense, some of these hobby channels can have little activities or contests. Our photography channel runs an inconsistently updated photography "contest", but mostly we just share our favorite shots and talk about gear we can't afford.
- Have a team happy hour about once every other week, during company time. We do ours on a Friday, just after 1pm for me (central time). It's late enough that many people are ready to relax a bit before the weekend but early enough to let the people on the west coast join. Just grab a drink (hard or soft) and talk about stuff!
- Do activities after hours with coworkers, if they're open to it. I run a bi-weekly movie night (on weeks we don't have the happy hour) and invite everyone on the team to come. Frankly, it's usually only a couple regulars, but those that come seem to enjoy it and it's really improved team camaraderie. On some nights we switch it for a game night. Among Us is a newer game that is absolutely perfect for this.