Yes. But try not to fall off. Be sure you can handle the extra weight when cornering (and make sure it's fastened to your body tightly!)
I'd get a front mounted bicycle basked to put the book bag in, then I'd get a guitar case with a shoulder strap and carry the guitar on the back while riding.
The fellow that talks about riding in the summer forgets to mention the swarms of mosquitoes big enough to carry a moose away. Because of all the insects when it's warm and because it's not much fun when you are cold, I don't think "common" is the right word.
It may be necessary to carry motorcycle insurance if the state your vehicle is registered in requires it. In any case, it is probably worth the relatively small upfront cost to carry motorcycle insurance to avoid any expensive liability in the case of an accident.
Plenty of retailers carry motorcylce covers. An example of one of these retailers is walmart. They carry a range of motorcycle covers that can be purchased in their store or via their website and then collected in store.
"He used to carry his guitar in a gunny sack, and sit beneath a tree beside the railroad track."
Carry it in your wallet
The question is very unclear.
There are a number of places where one might find a motorcycle windshield. Most motorcycle specific stores will carry them, as will custom motorcycle shops.
Stores that sell motorcycle parts and products usually carry these motorcycle ramps. One store that has is the Discountramps. They have motorcycle ramps at affordable prices.
It is perfectly legal to carry most knives while on your motorcycle in the state of Arizona. However if you intend to carry a large knife, like a hunting knife in a concealed manner, you will need a permit.
a guitar
In their wallet.