if you could send a cubesat or smallsat to any object in our solar system on a faster-better-cheaper/discovery-class sorta mission
where are you sending it, and what primary instrument(s) is it carrying, given how threadbare the mission probably has to be
Probably a flyby mission to Triton and Neptune; something similar to the Trident mission in trajectory and planning, but with an even more limited instrument selection. A flyby mission to Eris would be another good choice.
Assuming that I can basically only fit one or two instruments, the priority would probably be a modern imaging camera with a decent filter selection and a magnetometer.
If it was at all possible to incorporate a third instrument, then I'd pick a VIS-NIR imaging spectrometer.