Sure, while you're unemployed, job No. 1 is to look for another job. However, job hunting should not be the only thing you do while out of work.
You should also devote a bit of time and energy to doing something interesting that you can talk about at job interviews. Ideally, this "something interesting" should (1) enhance your employability and (2) keep up your spirits. Some ideas:
Start a blog. Writing a blog will make you smarter (hence, more employable) because you'll be doing all sorts of researching, reading, and thinking. A blog provides a showcase for said researching, reading, and thinking, plus it makes an impressive landing page for potential employers who are Googling you. You may also make some helpful contacts in the blogosphere.
Start a company. It's OK if it's a small one. Maybe your business won't end up supporting you, but if you have always wanted to be your own boss, what better time is there to try than now? You will learn tons. You will meet many new people (which is great for networking).
Volunteer. Yep, go out and do something for someone else for a change. It's good for society. It will give you a reason to get out of bed in the morning. And you never know whom you might meet.
Learn a new (job-related) skill. Now is a great time to take a course or certification class to beef up your credentials. If the cost is prohibitive, get creative—think volunteer, think apprenticeship, think barter.
Learn a new language. Being bilingual is a useful skill and looks impressive on any résumé. Again, you can do this on the cheap. For example, find a native speaker of the language you are studying who wants to practice English, and do a conversation exchange.
Lose weight/get fit. You'll feel healthier, have more energy, and present a better appearance at job interviews. What's not to like about that?
Do some public speaking. Join Toastmasters, get some skills, and get out there. Many clubs, associations, and classes are looking for speakers, especially free ones. Bonus: Your interview skills will improve, too.
Teach a class. If you're a receptionist, you can teach phone skills to immigrants. If you're an engineer, you can tutor high school kids in math. Not only can this be a very satisfying "hobby," but it looks darn good on a résumé. You might even get paid (a little).
Get an internship (no matter how old you are). "Working for free" can have many advantages. For one thing, an internship looks better on a résumé than a long stretch of unemployment. An internship is also a good venue to practice new skills, build a portfolio, and (where have we heard this before?) build your network.
Freelance/temp. This one has the advantage of bringing some money in—good for your morale as well as your bottom line. And, again, no matter how small the assignment, you will still be able to put "currently freelancing (or temping)" on your résumé.
You don't have to do all of these things. Just choose the ones that are the most doable or appealing to you. When you're at job interviews, you will appreciate being able to point to the productive, interesting things you do when left to your own devices.
Karen Burns is the author of the illustrated career advice book The Amazing Adventures of Working Girl: Real-Life Career Advice You Can Actually Use, recently released by Running Press. She blogs at www.karenburnsworkinggirl.com.