Many of the perrenial ground cover plants that they sell in flats in big box stores will fit your bill. Verbena grows quickly, as do petunias and allysum. All are easy to remove when you don't want them anymore.

You could seed a cover crop of flowers or even one of the more attractive farm-type crops. A nice thick cover crop would help control weeds and would control erosion. I think now is the right time of the year to plant, since there will be some rain to get things started.

If I was doing it, I'd seed sunflowers in areas a little farther from my usual vantage points, with something a little shorter in front. The cheapest way to get sunflowers is to but bags of black oil sunflower seeds sold as birdfood. I'd use a lot of seed and rake it into the soil. Burying at least some of the seed in the upper inch or two of soil, and scattering straw over the seed will help deter birds. For areas in front of the sunflowers, you might try alfalfa, cosmos, safflower, buckwheat. If you can find the seed in bulk (try the internet) Salvia coccinea is a great plant for this type of situation - it grows quickly from seed, flowers constantly, and attracts both hummingbirds (to the flowers) and goldfinches (who eat the seeds), plus it is fairly drought tolerant, so it won't take too much water to keep it looking good.

Spray the weeds with round-up and add more mulch--get some heavier stuff that the wind won't blow around.