As a more experienced intermediate-level cello player who still travels fairly often, I have gained confidence that I won't lose all of my hard-won skills and revert back to the beginning if I don't play the cello for a few days. But practicing is part of my life, something I do, and am, and I practice every day, whether I have a cello available or not.
I don't mean to write an exhaustive treatise on no-cello practice, but thought I'd list the things I brought along on my current trip, by way of answering Ten Northern's question, "What do you mean by 'no-cello practice'?".
1. String quartet by Alexander Glasunov, op. 26. I'm not actually playing this, but a friend asked me for some fingering recommendations, if I had time. Marking a part with fingerings and bowings is a great thing to do when traveling. This quartet has lots of little chromatic puzzles to work out those fingerboard geography muscles.
2. Brahms sonata no. 1 in e. I am preparing the first movement to play for T4- at my lesson in 2.5 weeks.In addition to marking fingerings, I'll totally mark this part up for memorizing, with bowings on almost every change and fingerings at the beginning of phrases in addition to on the shifts, where I usually mark them. That's so I can start anywhere on the right bow and finger when I am practicing tiny bits at a time, later. I will also survey the movement for structure and work out which parts are alike, and how the alike parts are different. Then I'll start memorizing it by singing it.
3. Duport #7. I have already re-written this etude as chords, changing it from it's written-out arpeggios into something I can see the structure of more clearly. With this, I'll mentally practice chord changes as coordinated movements. I find this quite fatiguing, so I'll only do a few measures at a time.
4. A chair. OK, I didn't bring that with me, but plan to use one here in the room. I'll tip it back toward me as I sit in another chair and practice one of my new motor skills. My directive is to develop more flexibility in my thumb mcp (left hand), particularly in thumb position. So I will pretend I have a fingerboard, and with thumb and one finger at a time observe what I need to do to vibrate with a flexible hand while keeping equal weight between thumb and finger. This is also something you can do with arm cello, but since I have a wooden chair handy I thought I'd use that instead.
I also have two books with me: Mastery: The Keys to Success and Long-Term Fulfillment, by George Leonard and Set Your Voice Free: How To Get The Singing Or Speaking Voice You Want, by Roger Love.
Here's a quote from this morning's reading from Leonard's book to inspire you:
...the essence of boredom is to be found in the obsessive search for novelty. Satisfaction lies in mindful repetition, the discovery of endless richness in subtle variations on familiar themes.