At the end of the year 1968 during the months of November and December, Paul McCartney was busy at Trident Studios/ EMI Studios (mainly Trident) completing an LP with Apple artist Mary Hopkin. He is listed as the producer.
Paul also plays on the LP (of course) with the following instruments: Tambourine on "The Honeymoon Song" which was also performed by The Beatles on the BBC; "Happiness Runs" written by Donovan and featuring Mary Hopkin on the acoustic and Paul on bass; both Paul and Donovan play their acoustic guitars for the songs "Voyage of the Moon" and "Lord of the Reedy River". Another track, "The Puppy Song" written by Harry Nilsson has Paul playing piano.
"Love Is the Sweetest Thing" features Paul on the acoustic guitar alongside the band. Paul also plays acoustic on "Lullaby of the Leaves". Finally, Paul plays acoustic guitar, bass and drums on "Prince En Avignon".
All of these tunes can be found on the Apple LP/CD "Postcard" by Mary Hopkin.