This is a situation where you don't necessarily decide what you want. Instead you go to a nursery or two and a big box store or two and see what they have right now that fits your specifications: Tall, sun-loving, annual, in bloom til October. At this point in the season, you really have to choose from what's out there. I bet you'll be pleasantly surprised.

Hmm..."only allowed to plant annuals"?...Isn't a perennial that dies an annual? Well not really but who's to know! LOL!
I agree with Ginny...plant what you can find...if it's still there in the spring....say "Well! Gol Durned! what a healthy plant!!"
Linda C

The pickings are slim right now. Better luck to me next year!

Linda, you are on the mark. Perennial? Annual? It's all relative to your zone. As some say at noon, it's happy hour somewhere! I did plant one of the lavenders I propagated from the plants my old beds. But, hey, some lavenders are annual in my zone, and they are almost certainly annuals in zone 4. How was I to know?

This reply is probably waaaay too late, but a Purple Majesty Ornamental Millet would fit the bill.

It's a tall, grass like plant with purple foliage and feathery blooms. We have one in a pot with million bells surrounding it and it's gorgeous. The blooms of the millet are about 4 feet tall.