You take the entire planet apart and use the entire mass. It contains large amounts of nitrogen, carbon monoxide, water and stony material. Use the energy of D+D fusion to do the job. Fabricate a small megastructure (compared to Niven's Ringworld or Dyson's sphere.) that never the less have much more floor space than the inner solar system.
I imagine it shopping malls for millions of miles as is shown in some O'Neill graphics and also Wall-E. Some people would find it comfortable.
It would be a big space and have some room for "nature". It looks hard for me to see how to provide relief like a mountain range unless the relief is axisymmetric.
I sketched out a very small Bernal sphere for a showy space hotel where it would be quite easy to climb up to the low-gravity "hub". It needs 15 starship loads of N2 for the atmosphere - pluto has a lot of N2 and could enable those bit O'Neill airspaces.