Some ideas would be to listen for, or create your own, long sentences with multiple clauses (even if shorter sentences would be easier. Also by speaking to different age groups, for example older generations tend to have many colourful expressions that are not commonly used anymore. Apart from that, if you wanted to get very complex you could ask someone to explain a difficult subject matter to you, something they might have expertise in, e.g. something scientific, their heritage, their thoughts on politics etc. Anything that forces multiple clauses and deep consideration and revision of ideas.

In a very basic way, with my less fluent students I continually ask then to explain their thinking behind an opinion... Why, why, why

And of course including more advanced material than their level dictates, to push them harder and learn faster.

Hope that helps somewhat!