I am not a big fan of the camelback type arrangement. The sippy tube always manages to find it's way into some scary places.
I put several bags (not baskets) on my bike so that I could carry a bunch of extra stuff (in addition to a small shoulder bag). This worked very well for me and enabled me to leave camp with over a gallon of water without feeling that weight. I didn't seem to have any trouble with security on the bags.
On my person or in my bag I had a water bottle, chapstick, small snacks, notebook and associated pens, and some other small stuff. Things needed for basic comfort and a light load so that I would not be tempted to leave it on the bike.
On the bike I had a whole load of stuff: more water, alcohol, energy drinks, more snacks, a jacket, my parasol, and a bunch of other stuff that got pared down as the week wore on and I learned what I really needed to carry.
Don't worry. You'll bring a bunch of stuff you don't need and you'll wind up receiving things from friends and wondering how you ever lived without them.