Huh, when I checked the image of four o’clocks I thought they looked a bit like Morning Glories. When I looked for the image of your dondiego de dia, I found they are Morning Glories! 
Good point. I keep forgetting that not everyone wants to putter in their yard for hours.
Let me amend it to, if you do make beds, make them a proper size. If you have two shrubs and a lawn, and that’s as much as you want to do in your yard, that’s fine too.
Am I correct that you live in Denver? Here is a list of Denver natives:
Growing from seed is cheap and low effort, but it’s not reliable. Lots of seeds need certain conditions to germinate, or need to be covered in exactly this ->| |<- much soil, or you’ll lose a lot to birds or rodents.
I’m in a different zone, so what has worked for me may not work for you…but my advice is plant lots of perennials on sale and see what continues to come up. I’ve never gotten phlox to work, despite planting it several times, but I have enough bleeding heart in my yard for six people. I’ve been able to divide out the twelve sedum, two hosta and dozen daylilies to fill a huge bed. Find friends or neighbors who garden - perennials actually like to be split - and say “I need easy growing stuff it you split yours out” (A friend of mine says “people actually BUY hosta? we get them in our neighborhood from boxes that say 'free to good home”). Consider the plants that are growing at old lady houses near you - the stuff that has been their for years - for my zone, its hostas, peonies, lilacs, snowball hydrangeas, black eyed susans, lily of the valley, ferns and purple coneflower - in other zones its poppies or azaleas. I know some of the lilacs, ferns. hostas and lily of the valley near me are older than I am (and I am not a young woman). Be patient - ten years ago I planted some columbine that I thought had died out…four years after I planted it, I started to see columbine - I now have a healthy bed of it six years after dropping it in the ground. And don’t get discouraged - a bad winter can mean something that has been going strong just gives up. Spent $100 or so on perennials ever year plus the ones you can get from friends and neighbors - until you have your beds where you want them.
My suggestion is alstromeria or Peruvian lily. You can get short ones (which I grow in containers) or long ones. They probably look familiar because you see them in every grocery store bouquet. They come in LOTS of different colours, and according to some cursory googling, will thrive in zone 6b. You may want to keep them in containers, as they are sometimes reportedly hard to get rid of!
OP, look for sulphur cosmos, which bloom in orange & yellow shades (as opposed to the pink color family teela mentions) as I’ve grown both, and sulphur cosmos is even tougher than the pink versions.
Re-seeded itself like it was about to go out of style, and one individual plants shot up taller than me, with the highest flower blooming at almost the roofline (1 story house, but still!)
Dies back each winter, but comes back from seeds reliably. Ditto for morning glory (ipomea) but those vines need something to climb. Doesn’t get heavy or bushy, though - you mentioned a fence they could ramble over?
Surprised to see recommendations for morning glory. I consider that a weed that’s hard to get rid of. Spreads all over both in terms of vines and seeds.
The wild ones definitely are weeds. Couldn’t even beat them while actively weeding a veggie garden–they seemed to grow faster than we could pull em. “Flower” morning glories are big & pretty, however. And quite persistent even without supervision.
Bear in mind that the common name “Morning Glory” can apply to “over a 1000 species of flowering plants in the family Convolvulaceae” [Morning glory - Wikipedia]
I’ve planted the commercial varieties of Ipomea and they are easy to get rid of if you don’t want them. It’s the more woody wild plants called Morning Glory(also bindweed) that are hard to get rid of.
Oh, god, bindweed is some sort of revenge plague against mankind. It’s like morning glory’s awful crackwhore sister - a mean, drunk bitch who overstays her welcome and eats all your food.
Named varietals of morning glory are polite, respectful ladies who bathe regularly and come to a complete stop at stop signs.
An old favorite is “Heavenly Blue” which is a clear azure color, and sometimes shows up at the Dollar Store or other cheapie sources for seed packets. I highly recommend that one.