Maintain a cadence of scheduling events at regular intervals. These will help you drive periodic engagement within the community.
Smart and quirky conversation prompts are interesting and super quick engagement drivers. Here are a couple of templates/suggestions you could use.
Gamification is a great way to identify and appreciate members who are driving engagement for your community. Here's a framework on how you can setup the leaderboard and use it to drive engagement.
Networking is the most undervalued proposition of any community. Helping users form meaningful connections should be on top of your mind as a community builder.