Label things as you go through your day. "Oh, look at that enormous skyscraper!" Talk about what things are called. Use great (and multi-syllabic) adjective to describes what you are seeing together. Encourage your child to do the same. Don't feel stuck on using kid words--be extravagantly expressive!
Besides naming things, use words to describe feelings. "Oh man, I am feeling a little weary right now. Let's rest for a bit and then continue." Or "I am sad today. I am missing Jenny a lot." Demonstrating that words can communicate feelings is so important for your child so he can learn how to express what he is experiencing.
Words are so powerful--in reading and in communicating. Help your preschooler learn to use and enjoy them!