A classic smallframe winnebago from the '70’s with appropriate pinstriping, decals, and flashing.
Maybe, a glass encased bus stop and a bench, the crosswalk buttons to activate, the emergency milemarker phones along interstates that call the Highway Patrol. “Indoor ATM’s”- the “front porch” of every bank. Didn’t the movie 
take the tardis to mundane, spectacular, and new heights?
A hippied-up VW van.
A photobooth?
A soda Vending machine. Yea I know you wouldn’t normally walk inside one but the ventire front of a typical machine hinges open so this would work well for the TARDIS door. I believe the Master’s TARDIS once disguised itself as a sarcophagus. I don’t think the TARDIS has to pick something that even has a door, just any object that blends in.
And of course you could make it a vintage machine.
Plus, sponsorship!
Does it still do that, though, now that it’s lost its camouflaging abilities?
I thought that was the idea - that, at some point in the 1960s in England, it had lost its camouflaging abilities, but happened to be something people could walk into, reasonably not be seen inside or taken away or seen as dangerous, and wouldn’t look too incongruous in most circumstances. I can’t really understand the latter reason, but the others work.
Well, actually I guess the natural lineaton of the movie “Up” would be Tardis, Jumanji, Zathura. To be honest.
…You could plop a house down in many Detroit neighborhoods and noone would even notice, nobody there to notice…
Kinda like the Paper St. House/HQ in Fight Club.
A large, beat up looking duffle bag. You open it, step inside it and sink down into it like a portable hole.
The revived show has mentioned and shown that the TARDIS has telepathic tech, not only does it automatically translate languages for the Doctor and his companions but it has some kind of ability to make itself not be noticed. Even the TARDIS key has this ability, one of his companions wears it as a pendant while a fugitive and it works.
So even with a broken chamelion circuit it still makes it hard for people to notice it.
It would be The General Lee. As for the need for running inside while someone shouts “Come on!”, you can accomplish it by doing the Dukes of Hazard jump-in-through-the-window approach. 
Driven by an elderly gentleman in a beige-and-white safari jacket, and a young man in a red sweater with a yellow collar?
(A reference to the 1970s Shazam TV show, for you youngsters out there.)
I got it. 
Port o Potty
http://irenefelder.files.wordpress.com/2008/03/portopotty.jpg
They’re already painted Tardis Blue. 
A refrigerator box would be tossed, and a cab might get towed if left for any amount of time on the street. If you are looking for something that is everywhere, I also vote for a Starbucks.
I like the idea of a transformer box or soda machine. Both are common enough sights that they wouldn’t raise too many eyebrows (unless they appeared in a really strange place) and are unlikely to be tampered with. (It’s too common for a car to be towed and impounded if it gets left in a strange place.)
Reminds me of the sinking entrance to the Cardiff Torchwood.
I came to say Coke machine too, but then I realized if someone saw you walk into it, they would definitely want to investigate; same with a tranformer box.
I would vote for a small brick square with a sign on the door that said “Employees Only.” It could attach to the outside of almost any commercial building, and be utterly ignored by all concerned. People going in and out need only wear a blue maintenance jumpsuit to make themselves almost invisible. Or at least only good people would notice them.
It could likewise land on the roof of any building, and the occupants could then come and go as they pleased through the main lobby.
What about one of those yellow tarp tents that the utility maintenance people put over their work sites? There’s been one outside my local grocery store for two weeks now. Maybe I’ll stop in. . .