Ryan, our Product Manager, and I started as we usually do, on the whiteboard coming up with possible solutions. We claimed a wall space and iterated and brainstormed every afternoon. One of our favorite frameworks was the Opportunity Tree. It kind of forces you to think of as many different possibilities as you can all leading to the same outcome.
Once we had the idea for an activity feed + journeys, we pulled in our engineers to discuss the MVP and how to simplify it. It's important to pull them in early for many reasons, but especially to help you decide the quickest way to deliver the outcome you're after.
User stories & mindmapping
I like to spend some time really getting to know the ins-and-outs of the project, so I'll write user stories with my product person. I'll also use my mindmapping tool to lay out whatever is on my mind. Sometimes it's the flow for a user, or different options, or information architecture...it really depends on the project. It helps me to understand the project more, because I feel like I've thought of all the user scenarios.