1) Make a practice plan! Consult your teacher - he will be MORE than happy to assist you with a practice plan that works for travelling rather than have you cancel your next lesson because you "couldn't practice". If you'll be using a rented instrument that differs substantially from your own, explain to your teacher that you may not be able to make progress with major rep, and ask for an assignment of core technique exercises. Your teacher has been secretly awaiting an opportunity to get you to work on core technique, and will instantly write out a list! Or, he may give you listening assignments, and he'll help you choose and learn a piece to play for Grandma and make her happy - then she'll let you have pie. This time may turn into a big benefit to your overall progress, so make the most of the change in routine.
2) If you'll be at home, or staying elsewhere WITH your own instrument, OPEN YOUR CASE and set up everything you need to practice as soon as you arrive at your holiday digs. You're much more likely to practice when your instrument is ready to play.
3) Don't hesitate to ask for a space you can practice - and get creative about what practice space will work. If you've ever attended a summer festival, you are quite familar with outdoor practice huts, so you can certainly practice in Grandma's garage or garden shed. She may hesitate to offer it, thinking you need a more civilized space, so be assertive about asking permission to practice anywhere you and your instrument will fit! At hotels, you can ask to use a meeting room. Or, head for a stairwell or parking garage - asking for forgiveness may be better than asking for permission, if you think your request will just confuse the personnel and they won't be able to give you a direct answer. Or, look into practicing at a local college (where the practice rooms won't be as crowded or heavily policed), a church, or ask a relative to let you into her office while it's closed for the holiday.
4) Skip the gratuitous post-Thanksgiving shopping excursion and PRACTICE while everyone is out! The day after the holiday, the household will start getting cabin fever and might want to head for the mall. You are quite capable of doing your shopping on a less-insane day of the holiday season, and your relatives don't have lessons, gigs, and juries coming up.
5) Bring your stuff! It's amazing how the car on a road trip manages to fit everything that's a high priority, so if it's at all possible to bring your instrument, absolutely do it. Speak up for yourself before the space gets committed. And if you're making a case for bringing your instrument on a trip so you can practice, make sure you actually DO the practicing, so you don't get overridden the next time you want to bring your ax along.
6) If you can't bring your instrument: bring your bow (get a bow case) rosin, endpin, mouthpiece, reeds, sticks, etc. and a few sheets of music you may need. DON'T BOTHER WITH: a music stand, entire books of music (folder, binders).
7) Be generous when seeking resources if you couldn't bring your own instrument. For example, if you decided not to bring your own bass or cello due to airline fees of several hundred dollars, anything you spend to rent one at your destination is free money - including a nice thank-you gift if a friend loaned you an instrument. Make sure to return anything you borrow in pristine condition and on time, understand that many musicians won't loan their personal instruments and/or have holiday gigs, and expect to be asked to return the favor when a friend travels to your town. Dig up some supplies to properly clean off the instrument, strings, and fingerboard.
8) The music store is your friend. Ask to be dropped off there while the fam is out shopping. The proper string shop will have a proper room to try out instruments (we do, naturally - in fact, we have multiple places to practice), and won't raise an eyebrow if you stay and play for hours. Here at Quantum Bass Center, we always have holiday visitors renting instruments to practice, but the rental instruments aren't as fancy as the ones you can play on all day in the store. Stay ahead of the curve and step out of the store's practice room when you see others need to use the space to evaluate an instrument for purchase (then again, you might find "The One" and be evaluating it for purchase!). Remember #6 and be generous - find something in the store to buy to show you appreciate their time. It's a great time to pick up holiday gifts for other musicians on your list!
9) If you travel more than once in a blue moon, consider an electric instrument that packs small and produces less sound. They really do allow for very productive practice!
10) If you're considering not practicing during a visit to relatives as you may be called upon to play something for them, plan for that now, and learn a piece they'll like to hear! Non-musicians are always curious about what you do, and will be gratified to hear one or two melodies. Be prepared to play something (if requested) soon after arriving, and, curiosity satisfied, they'll understand why you're sequestering yourself for hours on subsequent days.
11) If you really, truly can not manage to bring your instrument at all, make a playlist of all the material you're working on, bring copies of your music, and dedicate time daily to mental practice. This is so highly effective that it's prescribed by the best teachers, so if you have never done mental practice, make this trip your opportunity to develop it into part of your regular routine.