The Space Shuttle may seem like a boondoggle, with an amortized total cost of well over $1 billion US per flight. However, the marginal cost (the direct costs associated with one particular flight) is "only" around $60 million. Morever, since NASA will be retiring the shuttle in any case, the shuttle will be worth next to nothing to NASA itself once the last planned flight is over.
Note, however, there is one thing the shuttle can do that nothing else that exists or is on the drawing board can do. And that is to carry lots of people into orbit at once. With space tourism starting to take off, maybe one option would be to auction the fleet to the highest bidder who's serious about keeping them flying. That way, the potential buyer could save the R&D costs of designing a similar craft from scratch.
It seems to me there is a niche intermediate between the $20 million it costs for a week at the ISS and the 15-minute suborbital flights offered for $200,000 by Virgin Galactic. This would be the $2 million niche for spending the day in Earth orbit. What people really want is to say that they've been there and done that. Suborbital flights are kind of cheesy--you can't say you've really been to space until you've been in orbit. But the week in space at the ISS is overkill--and besides it costs 20 big ones, not to mention all that training you have to do.
So take the shuttle, and retrofit the cargo bay into a first-class passenger cabin. The bay is 60 feet X 15 feet. So you could have 10 rows of 2-2 1st-class lie-flat seats (seat pitch = 72 inches; seat width = 36 inches with 36 inches left over for the aisle). So, that's 40 seats at $2 million each: $80 million total for a $20 million profit per flight. The profit margin would be 33%--not bad.
The flight plan, as I envision it, would be to launch in the morning after breakfast. Once safely in orbit, the bay doors would swing open, revealing a panoramic view of the Earth below. The passengers would be protected by a seamless ceiling of clear plexiglass. People would be free to float around the cabin to stretch out a little--but there wouldn't be enough room for gymnastics. A gourmet lunch with champaigne would be served. The shuttle would do four 90-minute orbits before landing in time for dinner. After all, who wants to spend more time than that on an airplane.
The only showstopper I can think of: How do you get the passengers to their seats while on the launch pad without killing them?