Exterior: I'd do nothing except to remove the hokey centreline divider that runs down the front of the Bussard Collector grills, and get rid of the lit Thruster ports - the modellers made them look like more spotlights instead of like thrusters
The overall dimensions, hull scheme, etc are all perfect as they are.
Interior: I'd pretend the TMP-TSFS version of the bridge never existed. I never liked that gigantic jello-mould set, especially in all it's sickly beige-grey incarnations. TUC sets were a great nod to the TOS bridge etc, although I'd lose the digital clocks, and move the turbolifts back to the rear of the set as they were originally.
For engineering I'd probably keep something similar to the TUC set, but enlarged towards the aft of the ship with only one
PTC heading down towards the pylons, reminiscent of the TWOK set. I'd probably install some kind of "radiation room" opposite the Main Engineer's main workspace too. Sickbay would also be TUC-ified, devolving the TNG-style set. I'd keep a version of the TMP diagnostic sensor bed though - that was neat.
I always preferred the movie era uniforms; except that everybody suddenly became a redshirt
! So out with the maroon, replaced with a dark-grey/charcoal. I figure there's quite enough colour in the undershirts. I'd probably tone back the flared bell-bottoms a bit too - sometimes they looked a little excessive. Jackets and enviro suits were fine as they were (but again, replacing the maroon with a less-obtrusive grey).
STVI/STV phasers, communicators, tricorders, etc - with wristband communicators for shipboard use rather than having to run over to slap the nearest companel on the wall!