Here is a fairly easy tune, using just 3 notes
You may have played this before, perhaps on a recorder, perhaps in your LTP sessions.
You could:
Sing it to me, ray, doh
Sing it to finger numbers: violin = 2 1 0 cello = 3 1 0
Sing it to note names: F# E D
Play it
The track goes round 8 times so you have lots of chances. You could make your own mix of singing and playing, or get someone else in your family to sing while you play