7:30 is ok for a smaller race where you know parking (or metro) will be available, but if the weather is nice, you might want to give yourself closer to an hour. You'll be standing in line for bag check and the porta potty lines always take longer than I think they will.
Most races will say on their site if you'll get a clear bag at packet pickup. That bag can be used to hold your stuff. The packet will generally include your race #, a timing chip if it's using them, a race shirt and some promotional stuff like flyers. Occasionally there's food items in there. You might want to bring a sharpie and 4 safety pins. The pins are for pinning your number on your shirt, and they will usually have them, but occasionally they don't. The Sharpie is for writing your race # on your bag. they usually have those too, but if you have your own, it saves you stress.
What to bring: sweats or some other warm layer to wear over your running clothes while you're waiting. (then check it right before you line up at the start.) Also, I would bring an extra shirt for afterward (preferably long-sleeve if it's even remotely chilly) to change into if you're going to be standing around after the race - you get cold quickly in a wet, sweaty outfit. And maybe fresh socks.
I would NOT leave valuables in your checked bag, just in case they get lost or stolen. My water bottle holster has a pocket for my key, metro card and some cash. (Bring ID if it's a race that serves beer afterward. VERY important. lol.)
Headphones are a personal preference. I tend not to use music at races - I like to be in the moment.
Good luck!! Also, remember not to eat or drink anything unfamiliar on race morning. If you haven't used GU, Gatorade, or whatever, don't try it. Don't eat a huge breakfast if you usually don't. Don't drink caffeine beforehand if you usually don't. (it can make you pee more.)