What aircraft looks like a cross between a Cessna and a glider?
As you describe it, a motorglider would fit the bill. There are several types around, the most popular being the Grob 109 and the Hoffman Dimona.



Grob G-109 (picture source)



[Hoffmann H36 Dimona (picture source)

Other possibilities include the Valentin Taifun (the only one with a tricycle landing gear) or the Scheibe SF 36. All of those are of composite construction, but there are also earlier types with wooden wings and a steel tube fuselage, like the Scheibe SF 25 "Falke".

While a Cessna is a high wing plane, all motor gliders are low wing (well, except for this one here). Would a cross between a Piper and a glider describe what you saw even better?