Hi Jake -- and welcome to All Things Plants!!
Oh boy -- "instant gratification" are usually not words that seem to apply to starting new gardens... I take it you are planning a wedding in your yard??
I'd say pretty much any kind of sunflowers will work for you in zone 5b, so plant away... how "instant" do you need stuff, exactly?
Maybe you could do a gazebo with some fast-growing vines on it? Otherwise, i think a bunch of potted plants that are already growing, like geraniums, petunias, impatiens, would liven it up a bit? Maybe just get a couple of really dramatic plants and fill in with some smaller things? LOL, I think we do need a few more details here... whatever you do, I'm pretty sure you will have wonderful memories of the wedding!