It is a good idea to mix up your workouts. Doing different things benefits both your mind and body.
When you do the same workouts, your muscles become used to those movements and they don't have to work as hard because muscle memory allows them to adjust to the workout more quickly. By mixing it up, your muscles have to adjust to new ways your body is moving, which then allows your muscles to build in different and new ways. It can result in soreness, but make sure to stretch and keep working out...those things are most effective in reducing soreness.
Also, by doing new things, you won't get bored! When you get bored, you don't work as hard and you don't get as much out of your work outs. So mix it up to keep in top shape!
Here is a new workout to try:
Whether you are running, walking or on the eliptical, you can apply this. On an ipod, make a play list of slow and fast songs...about 20 fast workout songs to 7 slow songs. Put the ipod on shuffle, and go! When a fast song is playing, either run, sprint, power walk, up the tension on an elipitcal or increase the incline on the treadmill. You can mix up what you do during the fast songs, but make sure your heart is beating faster than normal, so you are pushing your body to do new things. When a slow song comes on, this is a recovery song. Slow down a bit, but not too much. Let your heart rate come down enough that you can carry on a conversation with someone next to you, but not for a long time. Go through this for 30 - 45 minutes. Have fun!