Quite a lot. The 68 changed the look of the steering wheel, different door handles, front seats have a latch to tip forward,
different vaccum line config. for headlight doors, different sequential tail light control, 68 had side marker quarter panel lights added.
Engines: 67 289 and 390
all with different h.p. and trans to match.
Just to name a few
There are hundreds of detail differences. Some show, some don't. The interiors in the XR-7 were largely the same except front seats and steering wheel as far as obvious differences but many parts were improved or modified in some slight way. The list of changes could go on for pages in a standard Cougar.