I don't think it's a terrible idea to hop to Epcot and back, actually, because (1) HS is going to be INSANELY crowded for Toy Story Land, so a few hours break might be useful to restore sanity; (2) HS is a half day park for us, and I don't see that changing too much post TSL; there is just not that much to do there we are interested in; (3) we prefer the HS entertainment (star wars or fantasmic) to Illuminations, so would definitely go back to HS for that.
That said, if you are wanting to go to Garden Grill for the characters, Hollywood & Vine is a nice character buffet in HS that has mickey and friends at lunch and dinner. Alternatively, you could hop to Epcot but in the world showcase near the international gateway, which would be less of a detour than going all the way to Garden Grill at the other end of the park.