@tomlukeywood either use Synaptic or type "sudo apt-get install fluxbox"
"*lxde" "*KDE" into a terminal window and then click on the icon next to your
username to switch DEs the next time you log in.
You may need to add a PPA for MATE, Trinity, Cinnamon, etc.; those are just
my own personal favourites that are right there in the reps. I couldn't get
JWM to work and Flux is close enough for my current purposes.
Sugar is also available if you have kids who would enjoy it, but I had
trouble switching back and forth with it and Heather Jr. (who is actually
male) likes the default DE best.