There are many options for Desktops below are a few
Some people may not know this but Ubuntu comes with Gnome and its easy to chose between Unity and other Desktops
http://www.gnome.org/
Now to install other Desktops it is pretty stright forward. below we will install the Xubuntu Desktop.
How to install Xubuntu and Xfce desktops
How to install Xubuntu and Xfce desktops
The first step to installing a new desktop in Ubuntu or any other version of Linux is to first update the system if you go to system settings then to details it will automatically check if updates are available. If there is any go ahead and update.
The next step is to determine if there is a package in the repositories if there is then go ahead and read the review and determine if the package is what you want then click the install it is just that simple.
Now I always look at the version number and go to the web site to make sure it is current . The Xubuntu-Desktop that is in repositories is current and that is great. We will just install that and give it a try. Now the other way this could be done is to go to the website and download Xubuntu and install a fresh version, I like the idea of changing at a whim. When the install is done its time to reboot and choose Xubuntu or xfce from the login screen the next few shots are showing more of the menu's.
On my system this ran without a single hitch. My Hats off to the people working on Ubuntu. The fact that you can choose different desktops just is amazing.