Pentax has their own sling bag. There was a recent thread here. It's a little simpler than the Lowepro and Tamrac bags, thinner material, but also lighter weight and cheaper than most of the 3rd-party bags. Personally I think most 3rd-party bags are over done to give the buyer the idea "this one must be better, look how thick the padding is, and how many pockets, hidden secret pockets, zippers, fasteners, and rainflaps there are!"
Anyhow, if you are looking to carry what you said in your original post, and want something simple, I can highly recommend the Pentax sling bag.
Edit: Just to clarify, this looks like a backpack, but there is only one shoulder strap, so you can easily swing the bag under your arm and rest on your chest/belly like a traditional droopy sling bag, to access your main compartment. It is quite compact.