This is an exercise I use with my clients who need help with bringing more color and clarity to their voices and speech. I use a couple of news stories, some wacky Monty Python dialogue, a couple of poems (one solemn and one nonsense), some emotion exercises and some children's books.
Reading aloud (and recording yourself, if you can) allows you to practice vocal variation and enunciation with different kinds of topics. Listening to the recording will let you hear where your voice and expression are strong and where you need some work.
Practice your volume -- loud and soft. Practice your pitch -- high and low. Practice your pace, sometimes speaking faster and sometimes slower. Practice pausing and silence. Practice putting emotion into your words. Children's books give you the most bang for your buck in the sense of using vocal variation to its maximum effect. If you have a child to read to, even better!
Reading aloud also allows you to practice and get better at sight reading, where your eyes move ahead of the words as you read and your reading becomes fluent and smooth. This is a difficult skill for a lot of people, yet we find ourselves being called upon to read aloud from time to time, and there's nothing worse than listening to a monotone voice reading one word at a time.
Reading aloud with good flow, expression, inflection and enunciation isn't easy, and sometimes it feels awkward, especially when reading to yourself. However, if your voice tends toward monotonous or if you have a hard time expressing yourself in front of an audience, reading aloud will be invaluable for the improvement of your delivery.
For more tips on improving your voice, check out these two books:
Speak to Influence: How to Unlock the Hidden Power of Your Voice and How to Take Care of Your Voice: The Lifestyle Guide for Singers and Talkers