"I couldn't practise, because there wasn't a piano where we were on holiday." My response would be "And your point is?". I get mystified looks. If they had at least taken their sheet music, they would have been able to do something.
Here is my view: practice is partly an act of the imagination, a bit like playing air guitar in your bedroom.
Frank Merrick (in his book Practising the Piano (OOP)), says that when practising, before you play a phrase on the piano keyboard, you should prepare in your mind how it goes. Even away from a keyboard, you could prepare how the phrase goes.
Also: the choreography of fingers can be secured away from the instrument - the kind of practice that is largely repetition of muscle actions. The performer needs sheet music, of course - or perfect recall.
The only drawback is that you don't get any feedback, since there is no sound, and that may be too much for some people, especially the young.
A story told by Andre Previn
I remember Andre Previn recounting a story of being invited to dinner at the flat of a Very Famous Conductor (could it have been Solti?), which was to be followed by a concerto rehearsal. After dinner, the VFC said, "I think we should rehearse now", to which Previn said "Certainly, but I don't see a piano, here." The VFC, then whipped the tablecloth off the table and said "I will conduct, and you play the piano part on the table." Unpromising! At the first piano entry, the VFC remarked that he thought Previn had played some wrong notes". "Ah", said Previn, "You're table is different to mine."
And a third thing, that practice should entail, but often doesn't, and for which holidays away from the keyboard are ideal: notice things!: go through the sheet music, focussing on the tempo, dynamics, affekt, - everything that isn't the notes. You will understand the music better, and be able to perform it better, and improve your inner ear to boot.
Apparently the technique is also practised by viola players: here is Antoine Tamestit (© Julien Mignot):