Next I usually add some annual color like some petunias or some Dianthus. This will add the color to the pot. This can also be changed with the seasons so that your pot never goes out of style. You can really add anything here and it does not have to be the same thing. If you see three to four different plants that you like, put one of each. It is a really neat to have all different plants growing together to make one look.
On the last level I add something that will spill over the edge. My most favorite is Bacopa. It has beautiful little white flowers all over it along with a deep green foliage. Sometime I just do hanging baskets of the stuff. Another really good one is Vinca Vine. It is a beautiful and simple green vine that puts little blue flowers on it.
That's about it. I guess I should mention that you would want to start with a really good potting soil. I like to use the Miracle Grow potting soil with food in it. I also use Osmocote as my fertilizer because you only have to apply it every 4 months. It is a slow release you can not burn up your plants with it. Also I add a bit of mulch to the roots of the plants to help keep moisture in. You can also use pete moss if you want a different kind of look. This helps keep from having to water so much.
To answer you other question, you probably want to wait to plant when you no longer are at risk of freezing weather. You could get a way of planting the ever greens. The freezing weather will not hurt them. In Oklahoma we do not plant annuals till after April 15. I am not so sure in Chicago but you can ask your local garden center and they should be able to tell you.
I hope I helped! If you have any more questions, you got my number.=)