Spend some time each month looking at other highly active communities. These don’t have to be in your sector, they just need to be highly active.
Look at the most popular discussions and consider bringing similar discussions into your community.
Below is an example from a gardening community.
Clearly “Your top tips” and “Your gardening pet hates” are the most popular categories. Both are self-disclosure discussions which everyone can participate in.
Once you have identified popular categories introduce them to you community. You might begin with a discussion thread “What are your pet hates about Fishing?” and expand it to an entire group/category if it proves popular.
Bonus: If you’re looking for whacky and wonderful off-topic discussions to stimulate visit AskReddit.