Let's see... I've been to California many times before, 9 times at SFO alone. Plus other layovers in LAX and SAN. But stepping outside of the airport and doing more than just maybe staying at a hotel? I can think of a trip to SAN, LGB (flew to LAX) for a day followed by SFO for a day, a trip to SFO, and I flew to SMF to take a side trip to RNO. So not quite "never visited" but a pretty big imbalance between layover and visiting. Plus, it took a while for me to actually do those trips.

To really address your question, I've got these cities on my "airport only" list:
SEA
OGG
PHX
OKC (unless you count a very short stretch/possibly dinner break on a separate road trip a while back)
FSD
RST (not counting driving through a few times since then)
CLE (not counting driving through at night several years before)
BTV
MAD
FRA (ok, I got a hotel here, took a bit of train/walking to get to)
LHR (ditto, but with a shuttle)

I can barely say I've been to, or returned to later:
PDX (flew to EUG, drove to PDX with friends, stayed the night and flew from there)
SLC (layover once, then training years later)
DEN now home)
OMA (had a few hotel stays here when driving DEN-MSP, plus dinner)
CVG (just a side trip for the day from IND)

Kind of amusing, I had a layover in CVG a while back, but their airport is in Kentucky. For years I was able to say that I had a layover in CVG, but have never set foot in Ohio. When I was in Cincinnati with the wife, we crossed the border in the car, and she commented that I still haven't technically "set foot" outside the airport yet. We were at a stoplight, so I opened my door, put my foot on the ground, then back in the car, and closed the door again. Tada! :-P