Meeting Bella.
She's simple to access, but because she shows up in an under-utilized part of the world, can be easily ignored. To start Bella's Questline, meet Katie at the Grassmarket any night after day 20 (the game will nudge you if you haven't done it). That'll give you a quick introduction to Bella, and her Pack. They'll become increasingly important as the game progresses.
You can then find Bella at the Grassmarket any evening after Day 23. Apart from agreeing to join you in Elsewhere (she's available as a Companion from then on), she also invites you to a night of drinking and strip-poker at the Gym. Wait until that night (it's a one time offer, so don't sleep until you've done it), and you'll get a fun night with Katie and Bella.