get a dry bag and bungee it to your seat, or get a backpack (i just heard a bout a brand called chrome that sounds pretty cool, probably expensive). if it's a long trip, you want it in something waterproof, and out of the way. i'm assuming you won't be using it frequently through the day so you don't really want it in a tankbag. I think you typically want to pack a tank bag with things that are used frequently throughout the day.
snacks/drinks, music, headphones, back up usb charger, ear pro; micro fiber cloth for visor, clear visor (if you want to run a dark one in the day), zip ties, map if you are stuck in the 90s :P, phone charger, maybe a few small frequently used tools, pressure gauge, chain lube, sunglasses, wallet.