Some barely secret aircraft, a whole lot of accounting and clerical staff, and a tremendous amount of security to make it seem like there are major secrets being protected there. That is what I would put at the world's most well known secret base, anyway.
This is the answer I got over drinks with a senior military leader. It's too well known to hold anything interesting. He contrasted it with Mount Weather and the Cheyenne Complex. Those are still the sites for continuity of government because there is too much infrastructure to move. The sites just got additional hardening as the locations became known.
It's a great way to keep the UFO nuts contained. Let them scurry around the surrounding area as much as they want. Occasionally arrest one who gets too insistent and the others will take that as confirmation that something big is hidden there.
Plus a bunch of old equipment and documents that are definitely secret and too valuable to just destroy, but no-one is entirely clear exactly how secret and valuable so it's safer to leave it guarded in place than to try and move it
That's what I think is there now. A big pile of stuff that isn't really useful anymore but they don't want anyone else to see. Indiana Jones warehouse style.
Or. Do a double bluff. Since it's so well known, put your actual top secret stuff there, since no one will really look at it, because it's so well known.