Honestly just raw manpower seems like something we could offer them. We can't send that many experienced soldiers equipped for battle, but I feel like a giant recruitment drive for devoted Ukraine peacekeepers could be sold if it came explicitly in exchange for security guarantees.
Before somebody says it, I'll absolutely go first, to whatever degree they'll take me. I really don't want to be part of America and we don't have a strong negotiating position, like the article points out.
Edit: Or even simpler, funding for their troops. We still have a healthy debt load and now's the time to dip into it.