Except for math (which has been everything from unschooled to AoPS), we've done all inquiry-led, cross-curricular projects for the last six years. We love it.
With dd13, many interests came and went. Others have stuck around for the full six years. She puts together a three ring binder at the beginning of the year for each project (or just adds another tab in an existing binder). It includes her goals for the year, a presentation plan and a list of resources - mentors, books, websites, moocs, etc. I've asked her to keep it to only four project areas at a time. Any more than that and we get overwhelmed. Sometimes she'll put one project aside for a term, pick up another, then circle back. Some of her projects have seasonality, so that influences things.
With ds6, his projects are still new and have a much shorter timeline. For example, he loves pop-up books. He made his own eight-page pop-up book, wrote a simple story in it and shared it with the local librarians. He loves board games. He designed his own and we 3D printed the game pieces. Stuff like that.
It is amazing what you can put together with a little creativity. I love researching resources for my kids (and their friends, and my friends, and myself). We've done things like beekeeping, birding, braille, bats... that's just the B's. Yes, I love external accountability (I work). We schedule our family holidays around their interests whenever possible. Ideally, I like to see reflection on personal growth, a group learning component, community service and a presentation or sharing of learning. Over the years, I've learned not to get too attached to their interests. And I no longer feel guilty when we simply can't support an interest - like llama farming. It has to be reasonable, achievable, flexible... and undergo family negotiations. We only have so many resources, which we all have to share.