Tomatoes, peppers and eggplant should be in the ground now. It is too late to seed them now unless you are planning to grow them in a greenhouse. If you don't have any transplants, get them at the store and transplant now.
We can, however, start some warm season vegetables from seed even now. These include corn, beans, squash and cucumbers. But again, remember, the varieties have to be the early ones.
Squash and cucumbers are usually advised to be seeded directly into the garden. I however, prefer controlled germination, in styrofoam cups, only because I know for sure which ones came up and also can keep their germinating condition closer to ideal, as in keeping them moist. When seeding cucumbers and squash, mind the depth. They should be only barely covered with soil, but the soil should be kept moist all the time. Replant them as soon as first leaves emerge and establish.
As far as the cool season crop, we have plenty of choices. All greens, including lettuce, mustard greens, collards, kale and bok choy can be seeded now. Again, it is up to you whether you seed directly into the ground or in containers first. If you seed into the ground, you will have to water that ground every day so that the soil keeps moist. That is the reason I seed in containers and then replant: I do not have an opportunity to baby the seeds because of work.
By all means seed all lettuce varieties now. Below is a picture of my own romaine lettuce seeds. It looks like a hairy mess, but really extra stems and other stuff is not a problem. It was not worth it to spend time cleaning the seeds to the commercial quality of presentation. The only trick with germinating your own lettuce seeds is to keep them in a refrigerator for at least a few weeks, so as to fool them into thinking that winter is over.
You can also seed all brassicas - cabbage, brussels sprouts and bok choy, as well as broccoli, carrots and beets.
I really like September. I can dabble in the garden in the evenings when it's not as hot anymore but still light, after work. I usually do garden work for an hour almost every day. Believe it or not you can accomplish a lot of things in just one hour.