Brass players can practice using just their mouthpieces, and drummers can practice with just sticks. What can a guitar (or similar instrument) player do to practice when they can't bring along their instrument?
It depends on what exactly you want to practice. I feel like I've answered this before so it may be a dup.
If you are a musician you can always practice your craft by singing. It may sound off-topic but it really is not. The ability to sing, and process short melodies in your head, is critical to playing any instrument. So, no guitar? Practice music rather than chops.
Get your hands on a small travel guitar. I have a Martin Backpacker that I love. I take it on trips, and pretty much everywhere. If I know I will not be able to rehearse for a while on my main set-up, this is a good substitute. They make other practice devices that are just a small finger board and strings. It keeps you active.
Along the same lines, if you are traveling for work or vacation, scope out music stores in the area. You can walk in and jam for a while on floor models. Most stores don't mind and you may get Youtube exposure.
In the worst case scenario where you can't do anything you can try some finger dexterity exercises taping out patterns on the table top. This will at least keep your fingers nimble.
There is quite a bit you can do, even if all you have with you is your mind.
Memorize the fretboard by visualizing it in your mind. You know, of course, how your strings are tuned. Using that, visualize (for example) where every G is on the fretboard. See the fretboard markers in your mind as you do this. This works well with flash cards: You write out all 12 chromatic notes on flash cards, and then as you draw each card, you see in your mind where those notes are.
Similarly, memorize where each chord is on the neck. For example, where is every G? As with individual notes, visualize the fretboard markers--see the fretboard in your mind.
Memorize the circle of 4ths/5ths. This is not specific to guitar, but very useful for any instrument.
Practice ear training. There are many ear training courses that are audio-only; these are perfect for listening to while you are driving, or any other time you want. There are also many ear training apps you can run on your phone. This is a good way to use time away from your instrument.
Google for "guitar practice neck" and you will see guitar necks, with or without strings, that are meant for practicing fingering. These are pretty inexpensive and small, and might be an alternative to your real instrument.
Youtube music educator Adam Neely made a great video on this subject, and one of the things he recommends is to use your right forearm as a substitute for the neck of the guitar (for righties). Here's the link of how you can do it.
It also makes you remember where notes are, as there are no fret markers on one's arm (actually, that would be a much better tattoo idea than what another recent question has as its premise!). To add, for practicing strumming, I use my right thigh as the strings, using it to mimic the resistance the strings normally provide. Of course, audiation is super helpful, and just mentally practicing is way better than not practicing.
For me, it's not just practice but development, so it includes theory study and a lot of reading and trying to understand why music works the way it does. I find the extra study helps a lot when I'm able to pick up a guitar. I'm interested in this stuff so it's easy for me to focus on more than just chord progressions and scales although I practice with a guitar regularly, it's not a serious problem for me when I'm away from it for a few days. There have been occasions where I haven't been able to play for a while because of physical injury, but I kept up with my studies and when I was again able to play, I noticed no serious loss of skill. That's my experience!
One technique that comes to my mind as well is mental practising. In case you allready know the piece of music that you want to practise, you need nothing at all, in case you don't know yet, you need either a recording that you can listen to, or have it written on some sheets.
You could begin just imagining and memorizing the melody (or in case you play chords, then imagining the chord). It may sound trivial, but it certainly is not trivial to memorize and imagine a certain interval (like a little or a big third, for example).
Next thing (and this is even more interesting!): Now that you have this particular piece of music in your head (may it be only 1 or 2 bars), imagine your finger movements. With each successing note, make it clear to your mind what wether it's an up- or a downstroke. With the left hand, you don't have a fretboard that you can place your fingers on. Yet you can think about which finger stays in it's place, which one moves one string ... and so on.
The result will be that you have an imagination of how it should sound, as well of an imagination of what to do with your hands. I already tried it (although on a piano), and it worked to some degree to play 6 bars of a piece of music I never played before, and of which I just had seen the sheets.
When I learnt guitar (far too many years ago), I didn't own a guitar. I drew out a fretboard on a piece of card, and practiced chord fingerings. There are some products out there that do the same things these days (e.g PocketStrings)
There are several pocket sized practice fretboards available from about $15-20. They often have four or five frets with strings so you can practice your fingerings. There is even one electric model for around $150 that you can strum and it plays the notes that you are fingering.
(Search "Guitar practice fretboard" on amazon or google.)
You can tap your fingers following the while listening music. I find that flipping a pen following the beat can improve your finger dexterity. I recommend to do it with both hand so you improve the finger picking and the other hand.
This is what I assign my piano students when they go on vacation (and want to keep practicing). I believe this will hold for guitar as well.
Focused comparative listening. Take some piece of music, and try to find at least three different recordings of it. If you're not used to this sort of exercise, as a first pass just try to hear which performance you like best. See if you can articulate why you like it. What is the performer doing differently that makes it special to you? You are just querying your own opinions here, so it is nearly impossible to be wrong.
Then, keep trying to see what other differences you can spot. Try to get into their heads and figure out why they made that particular musical choice.
A complementary exercise is to find a recording of something, get a copy of the score, and notate all the interpretation details you can hear in the recording. If you have a piano marking in bar 1, and forte in bar 15, what are they doing in between? Try to capture as much detail as you can. Make up your own notation if you have to. The intent isn't to mimic what they do, but to force you to become more aware of the choices you have as a performer.