A £220,000 project to create an eco-cafe, with toilets and an information point, was carried out at Burnham Beeches.
Superintendent Andy Barnard had seven years to repay the loan from the City of London and said he believed there was a business case for other parks managers to carry out similar schemes.
By replacing old cabins with the eco buildings, which feature green roofs, Barnard was able to increase the rent, which had previously been "peanuts".
He explained: "I had to have a very sound business case because the City of London set me a target of paying the money back in seven years. I got £20,000 straight back from the cafe in rent. Car park income we raise from donation machines. They look like car parking machines but visitors pay a donation rather than a fixed sum."
Barnard added that the new centre had helped to attract a wider range of visitors to the green space, including younger users.
The work followed a project to create a £404,000 eco-office for parks staff at Burnham Beeches. The building uses evacuated tube collectors for hot water on the roof, as well as being highly insulated.
"It has been a great reputation builder," added Barnard. "Now we are having meetings with decision-makers in a building that helps them take us seriously."