A note to the OP: if the hosts have pets, please choose non-toxic flowers. Lilies, columbines, aquilegias, daffodils look gorgeous in mixed bouquets, but can mean trouble for four-legged friends. I usually sort out the dangerous flowers and keep them in a vase in my bathroom where kitties do not have privileges of entry, but that means the integrity of the original arrangement is destroyed. Tulips are wonderful!
I live in the PNW, and I do bring hostess gifts (something consumable or flowers). I don’t expect that my guests would bring anything, but if they do, I assume that they do as a nice gesture and not out of obligation.