There are a lot of DEs in the repos you can install. Just install it and then
choose it from the menu when you log in (it's a drop-down menu in the
lower-left as you're typing your password). Some examples:
LXDE (Trisquel Mini): Install trisquel-mini and trisquel-mini-recommended
Xfce: Install xfce4 and xfce4-goodies
GNOME Shell: Install gnome-session and gnome-shell
KDE (Triskel): Install triskel and triskel-recommended
Unity: Install gnome-session and unity or unity-2d (unity-2d works without
3-D hardware acceleration)
If you're looking for lightweight, LXDE and Xfce are likely your best
choices.
