I went for almost 2 years without a diaper bag. To say it was nice doesn't even cover it LOL. Babies need lots of stuff. My awesome mother-in-law lives by the theory that mommies need something nice for themselves each time they have a baby (works for me), so she got me a Vera Bradley diaper bag. I had one with Luke but she thought I needed a new one (again, works for me).
I love Vera Bradley because if you are an organization junkie like me, you will love all of the wonderful pockets. You can put different things in each pocket and feel organized instead of digging through your bags looking for something. This diaper bag has a zipper pouch on the outside, a flip side with three little pouches on the outside, two end pouches, and inside it has three pouches on each side. It also has a matching changing pad. It also has a clasp to hook your keys to.
In my diaper bag, I use one end pouch for my wallet and one for a bottle. I use the zip pouch to put papers. I use the flip side pouches for my keys, cellphone, and hand sanitizer/tissues. I use the inside pouches for diaper cream (which we have never needed PRAISE THE LORD), those mini trash bags for bad diapers (we have needed a few LOL), pacifiers (we don't really use them, but I have them JIC), and nose bulb aspirator and saline drops. If Nathan is sick and needs medicine, I would put that in there as well.
In the main part, I keep a big stack of diapers, a travel pack (or two) of wipes, two extra outfits, two extra onesies, a couple of extra burp clothes, a bottle of water, a formula dispenser with enough formula for 3 bottles, and an extra bottle (sometimes I have more if I am going to be away from the house longer than one or two feedings. I would also put a travel bowl of cereal and a spoon if we would be feeding that while we are out. Sometimes I might have an extra toy or two if I don't want it on his carseat (it's not fun to sleep with toys whacking you in the head).
I always bring the diaper bag in the house after we go somewhere. My rule is: Go through the diaper bag as soon as you get home (or get a chance) and pull out dirty clothes, burp cloths, and bottles. Then replace what needs replacing (clothes, burp cloths, bottles, water, formula) and sit the ready diaper bag by the door. I know you could pack it before you leave to go somewhere, but I find that I am always in a hurry so this makes it easier. Plus if you have an emergency, you are ready to go.