Spider lilies would still be dormant in august and could be planted right away. They bloom after heavy rains in september & october (which leads to the other common name of hurricane lilies). They aren't super cheap to order, but if you find someone who you know that has a big patch they might be willing to part with enough to give you an wedding gift of sorts. When they're happy they multiply freely, and they are old passalong plants, so you tend to find them in older neighborhoods. Lycoris radiatii. They are pretty bright red balls of long elegant flowers that are summer dormant, bloom in fall, and have strappy liriope like foliage all winter.

Also, you maybe could find fall blooming crocus but they aren't cheap, either. If you could find them, tulips would just need to be refrigerated, as would any winter/early spring blooming bulb until around halloween/thanksgiving.

Seeds would be another wonderful idea. Ones that could be planted in the fall for spring bloom around these parts are shirley poppies, and somniferum poppies (breadbox), nigella, larkspur, bachelor's buttons, and sweet peas. All except the sweet peas are simply scattered on top of decent soil- they sprout in the fall, overwinter as tiny plants and then grow & bloom their heads off when it gets the least bit warm. The sweet peas need to be soaked and individually planted in the fall near something they can climb in the spring. Congratulations and have fun with whatever you decide!

There are tons of perennials that would love to be sprinkled on the ground in late August to overwinter outdoors and sprout the following spring. You could probably find a nice wildflower mix somewhere (WyattQuarels in Garner? however it is spelled).

If you want something that will jump up and bloom after sowing you could try Myosotis - "Forget Me Nots" (appropriate for the situation) most seed packets won't list the species which is important, only some of them are annuals, most are bienniels meaning they won't bloom until their second year - there are 50 species of them ; or try Malcomia maritima "Virginia Stock" which is a cutie little wildflower type thing that blooms 30 days after sowing usually in shades of pink to white.

tamelask and trianglejohn, thank you so much for your help! That is exactly the information I needed.

I think I'll be getting some seeds for the wedding guests, maybe a few different kinds since several of those look great (shirley poppies, larkspur, and forget-me-nots are at the top of my list right now).

I am also going to track down some spider lilies for my own yard. I had never heard of them and they are beautiful!

Maybe not what you thinking
but this is what my nephew and
his wife did for there June
wedding

Blueangel

Here is a link that might be useful:

We had an August wedding too. We made little herb planting kits for wedding favors. We bought two inch pots, a variety of herb seeds, potting soil, small zip-lock bags, tulle, and ribbon. We printed miniature seed packets and put a few seeds in each and sealed. Then we put potting soil in the small zip-lock bags. Finally, we put the seed packet and soil bag in the pot, wrapped in tulle, and tied a ribbon at the top.