The various Pennisetums are highly favored by florists. Pennisetum villosum, Pennisetum orientale 'Karly Rose' in particular are terrific. I like to let them grow in the ground (in a sunny spot with some water)for a season or two until they are floriferous enough to not mind having some of their flowers pilfered for arrangements.

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Giant Foxtail, AKA Setaria faberi, is an invasive annual, but if you harvest the 8" long seedheads/stems before the seed ripens, you get no re-seeding, and it's a gorgeous big seedhead. The plant will hit 4-5' in height in just one season, from winter sown seed.

Chasmanthium latifolium (Northern Sea Oats) has fabulous flowers for floral arrangements. Can be cut and used green or tan.
For curiosity value the seed heads on Carex grayii would be quite good in a bouquet. Neat looking, like a tiny mace.

Eragrostis spp. ~ any of the weeping love grasses have lovely airy panicles that are long lasting.