I have been on the search for the perfect park bag ever since our first trip in 2006. I've used everything from a backpack, to a Vera Hipster, and a sling bag. Nothing seems to be exactly what I want. I even bought 2 Baggallinis within the last month, and had to take both back because they were not big enough. I started to order the Baggallini Messenger, but I knew it would be too big.
I finally found something that I think is going to work. Just happened to stumble upon it on the Walmart website. I ordered one and received it, and my boyfriend liked it so much he got it. I was going to order another, and decided to order one for my son and one myself, since they were so cheap. It's very roomy, with all sorts of pockets and I love the outside water bottle pocket. It's called the Goodhope Bags Smart Sling Briefcase, and it's just the right size - not too small or big. I might even use it for my carry-on for the flight. My boyfriend and I got the black, and my son got the blue.
http://www.walmart.com/ip/Goodhope-Bags-Smart-Sling-Briefcase/16944258
I like this one! Plus it's not super expensive. I'm just looking for something small but it Must have a place to put a bottle of water.