I would consider not only financial assets but also any objects of sentimental value. If you have anything at all that you would want to go to a specific person, I would write a will.
Also, I would strongly consider spelling out in a legal document your wishes regarding medical treatment should you become incapacitated (i.e. draft a “living will”). For instance, if you get in a
car biking accident and are pronounced brain dead, do you still want to be hooked up to a feeding tube? Unfortunately, I believe laws vary from state to state and can get very technical (for instance, providing food may be treated differently than providing water). If you want it done right, I think you're going to have to talk to a lawyer.
Finally, it's a good idea to sit down with your family and make sure they all clearly understand your wishes.