The Prince of Wales and Duchess of Cornwall were presented with an Italian harp for the royal harpist at the Llangollen International Eisteddfod.
Prince Charles meets children at Clwyd Theatr Cymru in Mold
Charles and Camilla made a surprise visit to the folk music and dance festival which is playing host to performers from over 50 countries.
Temperatures soared to 31C as the couple arrived in the main pavillion.
Prince Charles said: "I'm extremely grateful that neither myself or my wife have to dance on this stage."
"This year is hotter than I would have believed possible."
The couple arrived at the eisteddfod - which is celebrating its 60th anniversary - at around 1445 BST.
They were met by dignitaries, including local AMs, MPs and the festival's president Terry Waite.
The couple signed the visitors' book before making their way to the packed pavilion, where 4,500 people were gathered.
He was presented with a £150,000 harp by harp-makers Victor and Julia Salvi.
Made in Piasco, northern Italy, using 500 pieces of gold leaf, it will be used by royal harpist Jemima Phillips.
She performed for the royal couple, including her own composition, which she told the audience was her "own version of God Bless The Prince Of Wales".
Prince Charles told the crowd: "It was six years ago that I managed to revive the position of harpist of the prince of Wales. My intention was to honour the great tradition of music-making with the harp in this country.
Prince Charles views sapling cultivation during a visit to Bellan Farm, Powys
"I also wanted to enhance the reputation of Welsh culture and music around the world."
The prince thanked the hundreds of volunteers who worked behind the scenes at the eisteddfod.
He said: "They work tirelessly, supported by a small team of dedicated staff who beaver away all the year around."
The prince, who was made eisteddfod president for the day, then joked: "The eisteddfod is of course celebrating its 60th manifestation this year - only marginally ahead of its patron."
When they left the pavilion, there were rain clouds forming and a rumble of thunder.
The couple then met the Saudi Arabia's UK ambassador Prince Mohammed bin Nawas Al Saud in the Saudi Arabian tent at the festival.
They ate dates, in a traditional welcome.
Earlier, the prince and duchess were entertained by 60 children at Clwyd Theatr Cymru in Mold.
They toured three workshops, part of the Arts and Kids projects, devised by the prince, to encourage young people's participation in the arts.
The prince also visited the Pontbren rural care project in Powys, an initiative by a group of 10 neighbouring farms near Llanfair Caereinion, looking at how sustainable farming methods were protecting the countryside.
The royal couple are due for further engagements in mid Wales on Thursday.