Principal Engineers work on projects that span the entire organisation, usually addressing cross cutting concerns of several teams. They advice Directors and VPs on strategical, mid/long term technical decisions. They set the standards of quality for the org and engage with the community at large. They work on extremely complex and highly ambiguous problems.
In a chat with a PE he told me that this is the first role where he wasn't told what to do. In that sense, PEs often proactively look for problems to solve or new initiatives that will have a large positive impact in the company, rather than work in items assigned by others.
Lastly PEs are expected to have strong communication and interpersonal skills. They should be a force multiplier, align teams and handle scalations.
PEs rarely provide new ideas in design reviews (our product teams don't need help with that) but often will strike things down or modify existing ideas to fit in with other products, for business reasons, or perhaps because their human management counterpart doesn't like the idea.
Alongside, they usually have some variety of R&D style side projects that they pursue according to their interests/specialties.