Jump in the car this morning (~55deg), fire it up, let it warm up for a min. Push clutch in, car won't go into any gear. Nothing grinds as I try to put it in gear, but I can feel the car lurch ever so slightly. Shifter moves freely left and right, I just can't push it into any gear.

A little more background. The past few months, the clutch pedal would "change feeling". Sometimes when I would drive, it would be a little more firm than other times. Sometimes, a little less firm than average. Now, I'm not talking huge differences that would greatly affect or change the way I would have to drive, but it is definitely noticeable. Maybe there is a clutch adjustment that needs to be made?

Car is a '91 300zx TT, ~100k, daily driver while the weather is nice.

In conclusion I would:
1) Inspect fluid level and cleanliness of fluid.
2) Inspect master and slave cylinders and lines (make sure they are not
leaking).
3) Bleed system with new dot 3 fluid.
4) If any of the cylinders are leaking I would rebuild.

I'm not sure if there is a way of checking a clutch besides driving it but I'd save replacing the clutch until the end.

Good luck with this. Let us know how it goes.