Ubuntu 13.10 2D
Solution 1:
Unity 2D has been discontinued since 12.10 (I believe).
If you're after a lightweight desktop environment, consider XFCE or LXDE (which is lighter than XFCE).
You can take a look here for more desktop environments for almost every criterion:
- What kinds of desktop environments and shells are available?
Solution 2:
You cannot use Unity without 3D in Ubuntu 13.04 or 13.10, no.