You may want to consider whether your trunk has the ability to be used as a cooler. It's a real treat to be able to pull a nice cold beverage out of your pack for a little refreshment. You could always jerry-rig your trunk to be used as a cooler by using ice packs and added insulation if needed, but I believe some trunk packs have been purpose built with that in mind.

I picked up a smallish soft sided cooler at Goodwill and sewed on some nylon webbing with release buckles. I use that when I think I might stop by a store for something that needs to be kept cold for later.

As far as handlebar bags go, I'd recommend something with a quick release system so it can be quickly put on and taken off. This is a real nice place to stick your camera, wallet, and any other valuables because you can easily pop it off and carry the bag with you.