With very little planning time needed you can have students completely engaged in reading online! When you visit Owl Ready to Learn (topic: books), students will see several pages they can visit for online reading. Give them freedom or choose the stories you want them to read, it's really up to you! The point is, you can easily make these books part of your "listening" or "read-to-self" centers. You also could partner students up and make it a "buddy reading" activity. All you need to get started is the link above, a couple of classroom computers/iPads, and headphones (optional).
You also could try using some of the great phonics/spelling sites, such as Starfall, to practice "word work."
Or, sign-up for a classroom membership and have the students publish their work on sites like Kidblogs and Storybird as part of your "writing center."
The possibilities are endless and I'm sure I won't be able to resist sharing more of these ideas with you later! In the meantime, I hope this gives you some new inspiration for planning future classroom centers!