Not every developer lost money as the Denver real estate market went sour. When Eric and Steve Holtze saw an apartment glut coming, the brothers went on building complexes with pools and racquetball courts. But they started a new company, Executive Residence, of Aurora, Colo., to rent out about half of their units for stays as short as one night. A one-bedroom apartment renting for $59 a night fetches about $1,220 a month, compared with $475 for a comparable but unfurnished apartment. They found customers -- mostly relocating executives and consultants who visit the area regularly -- through telemarketing and calls at local companies. The Holtzes' vacancy rate is 2%, compared with 12% of apartments locally.