If you live in a zone that can grow them, Rough Blazing Stars are just amazing at attracting Monarchs.
I know they are native to the Wisconsin, Michigan and Illinois prairies, but there are probably more states where they grow. The plants can be purchased at retail outlets that carry native plants.
I have several of them and it never ceases to amaze me when I go out and see 4, 5, even 6 Monarchs on a blooming plant.
Monarch Waystation #2116
Shelter from the Storm
Green Bay, Wisconsin