Good baby massage songs have a combination of firm but gentle, stroking rhythms, with simple words.
The words needn't be particularly related to the massage. A lovely massage song I use is Lavender's Blue Dilly Dilly. Sing it slowly and repeat the first verse as often as you like rather than struggling to remember all the verses (though they are all lovely).
Lavender's blue, dilly dilly, Lavender's green, When you are king, dilly dilly, I shall be queen.
Another great example of a song with simple words - there's just one! - combined with a soothing, regular massaging rhythm is Soualle. That one word is repeated over and over to the most lovely melody. It perfectly supports baby massage by giving you a great stroking rhythm, while your voice and the melody hold baby's calm attention.
I start a massage session with a little song called Face Trace. Use it to focus baby's attention on you and to set a peaceful mood as you gently name each part of your own and baby's face. To finish, have a giggle and a tickle with something like This Little Piggie Went to Market - but taken very slowly and gently, it's also a good accompaniment to stroking each one of baby's fingers and toes.