While crusing around the country side is a fun way to build time, I used the following scheme to build my time. It's called the check as many squares as you can per flight hour program.

Fly at night
Fly under the hood
Fly instrument approaches
Fly to various airports

I did this with another CFI in simple IFR planes, complex planes and twins. We split the cost and flew after our last students. Even though you're not a CFI, you can still split the time with another pilot.