When a padded envelope arrived by post at the Lake District Visitor Centre little did staff realise it contained a cache of cash.
Along with £250 in notes, the anonymous sender enclosed a postcard showing Dora's Field in Rydal and asked that the donation be put towards a project.
Centre manager Neil Manning said Brockhole, on the shores of Windermere, attracted hundreds of thousands of visitors a year and was a much-loved destination.
He added: "We are staggered by the generosity and going to put the money towards our playground restoration fund. Hopefully our secret benefactor will find out that countless children are going to benefit.
"Obviously sending that amount of cash in an unmarked, unregistered envelope - postmarked Preston - was rather risky, but the thoughtful, kind gesture has touched us all.
"All we have is a short message, saying 'following fundraising, please find enclosed a donation to be used for a local LDNPA project'. As it came to Brockhole, we thought it appropriate to spend the money on a local project here.
"In line with many other organisations, our resources are tightly stretched so to be given a boost like this is extra special. We have no way of saying thank you, but hopefully the sender can be assured the donation will be well spent."
Neil added that if anyone else was considering contributing towards the national park he urged that cheques be sent by registered post. Information is available on LDNP's website on www.lakedistrict.gov.uk/donating