I don't see my thoughts expressed by anyone here so perhaps I can chip in! I'm currently a graduate student but I was once that kind of bright bored student you are talking about. I had my fair share of boredom, lost motivation, exciting learning, disappointment and enthusiastic crankery. I can tell you that taking more interesting courses and finding good books to read will happen naturally - you don't have to help too much with that. Of course you can keep it at that but there's so much more you can do. Perhaps it won't take more than a couple of hours a week.
If the students don't need more courses or books what do they need? They need a mentor and a friend. They need someone like themselves but who has achieved something in life (If you are a professor that's probably enough success right there) despite difficulties.
They need a mentor to help them become a better student. They may be bright but how is their work ethic? How is their stamina and long-term motivation? Does the student know how to keep working on a problem day after day AFTER they've lost the motivation? Does the student know how to openly complain about their difficulties? Those are very necessary skills that they are not likely to develop naturally. An hour a week can make a huge difference for the student over the course of their undergraduate years and it probably beats answering another few e-mails. Not to mention that you get to help shape a promising student into something more than they would become on their own during the natural course of their studies.
On of the most important things a mentor can do is to take initiative. Schedule to meet each other but not in your office. The office, the desk and the chairs are barriers to communication. With these things you are an authority figure and they'll have a hard time relating to you well. Meet elsewhere and eat lunch together. Talk about stuff - ideally it's not going to be just about math the first few times. The more real and honest you are about things, the better it'll be.
Bright students need to hear your stories of how you had difficult times or how you tried to prove insert-big-open-problem-here at some point before you even had the tools to work with. They need to hear about procrastination and what they can do about it. Ideally, you'll share some memories of failures past and what you did to cope with them. Hopefully the student will be inclined to share their personal story as well. You might even have trouble shutting them up. If the student complains to you about their difficulties you are on the right track. You'll be someone they can talk to and confide in when the difficult times come. This is especially true if they end up doing some sort of undergrad research and even more true if they go to graduate school later on.
If they have a good time with you at your meetings they will want to come back for more and they'll want to learn from you and perhaps become more like you. From there on it won't be difficult to inspire them and teach them how to become better mathematicians.
Unfortunately my experiences with professors have rarely been this good and I only know about the things I'm sharing with you because of my own struggles as a student and because I've had amazing mentors outside of academia.