Card sort tests are one of my favorite ways to have conversations about information architecture, and find commonalities within large applications. It's also a useful tool for discovering system duplication and cleaning up "junk drawers" that tend to become dumping grounds for all sorts of settings.
Flows & Storyboards
Every designer needs to consider the end to end flow of their products. These happen at a higher level in my process, usually low fidelity sketches that remind me of the scope of the entirety of something before creating wireframes or mockups.
Conversation & Ideation
The other part of my thinking is unstructured as possible. Rough notes and doodles on whiteboards, sketchbooks, and devices to think through or sell an idea to or with stakeholders. Often with other people in the room, or sitting next to me.
Wireframes are a typical low-fidelity way for me to explore an idea or concept. In this case I've included a massive amount of wireframes so you can get a sense of how my typical airframe fidelity can vary widely depending on how much detail I need to validate layout and typography ideas.