Some of the nonhybrid rose varieties are about as vigorous as weeds.

Maybe you have space for one or two of these?

For summer reseeders, i'd do cosmos- if you like pinks, reds & whites, the sonata mix, or if you like oranges & yellows, the sunset mix (or both). They reseed happily if you don't mulch too deeply, and are great cottage garden types. Bidens is also another one, as is perilla. Oh, and brazilian bachelor buttons. Echinacea is a wonderful, no care perennial (the species), as is gaillardia aristata. Rudbeckia triloba is a short lived perennial and a trooper for me and reseeds itself around. They all have that billowy, blowsy bloom & shape that screams cottage garden. A butterfly bush or 2 would give you color for a very long time.

For fall sowing/spring color, try larkspur, shirley and/or breadbox poppies, bachelor buttons and nigella. Another short lived perenn, aquilegia canadensis reseeds really nicely for red & yellow spring color.

Woodsworm,
I hate to burst your bubble, but you want a raised bed garden in full sun with no water?
What is your plan for water?
You can't grow anything without water.
When you figure out how to get your watering devices closer to your new raised bed, I have some suggestions for you.
Good Luck!

Actually, I can schlep a watering hose out there, but I don't intend to depend on it. There will be a plastic liner stapled to the inside of the crossties, and I am a VERY HEAVY mulcher. It does rain sometime.

If you're a heavy mulcher your reseeders won't reseed. You'll have to collect the seed & sow them yourself a little before you'd want them to sprout. I hardly used to mulch and had great reseeding; now that i'm doing mulch at least 1 or 2x a season i've lost a bunch of my reseeders.

oh, and i have a raised bed (or it was when first built) in as full sun as i get (6 hrs, hottest part of the day), and i never water that bed and all the things i mentioned do well in it. You will have to water to establish the perenns, but other than that you shouldn't have to that much watering. Stuff will look more lush with it, but if you seed heavily it'll fill in and be fine, esp with lots of mulch.

Woodsworm,
Since you do have access to water for the bed, here are some suggestions, and most of these will do fine with little watering.
Russian sage, purple flowers, taller, perenial, light green foliage,
Dianthus- light green foilage, pretty reblomming flowers and they have new varieties out now. THey spread.
Yuccca- I have one and I love it! Mine is yellow and green and looks awesome, no maintaneous.
Ice plant- spreads, cool looking flowers.
Iris- you know what they are, I bought rebloomers at HD and I get bloom beared blooms Spring and Fall.
Gardenia- really doesn't need alot of water at all if you mulch well, Chuck Hayes is awesome and cold hardy with double flowers twice a year.
Verbena- mabey needs to be watered once a week real good.
Reseeds, but is easy to grow from seeds in spring.
Good luck to you, and don't forget to come back and show us what your bed looks like, I would love to see it.

Autumn joy sedums, russian sage, new gold lantana, coneflower, mystic spires salvia, butterfly bush, knockout rose, rose of sharon, daylilies and firewitch dianthus are more than enough to fill the space and give a nice cottage garden effect. Plant lots of springtime bulbs in there too and once established it won't need much water.

OP, before you go spending $$$$ on planting this bed understand that in that floodplain soil are a plethora of weed seeds that have to sprout before you can kill/remove them.

So much easier to let the soil set for a year and settle and then see what sprouts so you know how to deal with it before setting out expensive plants.
I know..all that work moving the soil and you want some return from your investment of labor but you don't know how laborious it is to try and pull noxious weeds from between new plantings.
Flood waters bring oddball weed seeds and tree seeds you may not notice growing nearby.

She could solarize it this winter and plant in spring. Or, if she mulches heavily before she plants or right after, that should cut down a lot on the number of seeds germinating since most require sunlight to sprout.

These are lovely suggestions; I'll take all the advice I can gt.

I lost a pine tree about 6 years ago and made a similar-sized cottage garden in the sole sun-space it made in my yard. I have had fun with reseeders that don't take too much water but have dragged a hose during some drought weeks.

Here are some additional water-thrifty and long-blooming suggestions: melampodium (reseeds), salvia coccinea, both the pink-and-white and white reseed, tall ageratum (watch this one - can take over but rips up easily enough), white and yellow, red four-o'clocks, belamcanda, dianthus velvet n lace and sooty, agastache rugosa, rosemary, lambs ears beside crimson pygmy barberry, coreopsis superman, lantana chapel hill, daylily pardon me, happy returns, feverfew, vinca, chrysanthemum, penstemon iron maiden, bellflower (platycodon), black and blue salvia, and Johnny's has "gruppenblau" salvia farinacea seed that I planted 5 years ago that has been perennial here in 7A.

I did try larkspur, corn poppies, batchelor buttons last fall at Thanksgiving and had low germination for the amount of seed spread but not covered - thought maybe a small amount of mulch might have helped but from Tamelask's comments, maybe not!

These are great. I want to 'solarize' this winter and so am not putting anything in now. By chance, has anyone had experience with solarizing in the winter? I may start a new thread on this. Thanks all!

I don't understand the concerns about weeds unless you don't plan on mulching which in that case I would use Preen. I have a drainage ditch that I use it in and it keeps it weed free.

I spray for weeds whenever they show up and get them while they are small. I also heavily mulch and weeds do sprout on top of mulch.
That's it. As far as preparing a bed, I spray it several times with round up to kill everything, wait a few weeks, ( during which more weeds will have sprouted!) then do whatever I want to the bed.
I rarely till but sometimes I do. It doesn't make that much difference. As long as all the weeds are gone when I start, that's fine.

I have cleared some heavy duty brushy forests and eventually turned them into weed free areas.
In those cases, with huge viney weeds and brush, i sprayed with Brush be Gone several times and waited six months before staring to plant.