Okay, so, you are going on a trip to wherever - let's say, the Smokies. You KNOW that there are wide vistas of misty blue mountain ridges, flowing streams, primitive log cabins, sunrise and sunset possibilities and waterfalls. You have, let's say again, a 24-105 mm lens. All those scenic possibilities can be well-covered with that lens. However, you know that there are also wild animals there - and they are oftentimes very accessible. Do you REALLY want to be like a tourist and get up close to that black bear with your 105 mm lens? If you don't want to get close, (and I seriously suggest you don't) are you REALLY going to be satisfied with that tiny blackish blob that will show up in your photograph when you shoot that bear from a safe and respectable distance? I can tell you, the answer will be NO! "Well", you say, "I can't justify (or afford) a long lens because I just don't shoot with them that often". You would be correct. So what can you do? You WANT a bear shot. Or a turkey, or a deer, or a coyote shot. I have the answer! You RENT a lens!
Renting lenses is a great way to have the proper equipment on hand when you go to a special place. You may never go out and need a 500 mm lens in your usual day-to-day shooting. But, you can have one for a few days or a week (or more) and get the kind of pictures you've always wanted. There are several good lens rental options. One I have used is www.borrowlenses.com. Another I have heard about is www.lensrentals.com. I'm sure there are others. You need to make sure you reserve the lens you want far enough ahead of your trip to ensure the rental place HAS the lens you want. Obviously, they can't have hundreds of lenses in the size you want. Plus, you have to compete with others who may be shooting at the same time. So reserve the lens you want ASAP. Also, most of the lens rental places have insurance in case something happens to the lens. GET IT! You don't want something to happen to your borrowed lens and then end up paying for its replacement. 500 mm lenses run upwards of $10,000. Other good lenses upwards of $1,000. Neither is really in your budget, or you probably would have had them by now, right? Get the insurance! Also, return your item on time. There are penalties when you don't. Yes, it's a bit of pain to make sure you box it up right (they provide the packaging and the return label so your packing is secure) and get it to the FedEx place on time, but it's cheaper than buying a new lens. Remember, too, that they usually rent cameras, as well.
Now, this is probably going to be more of a challenge for you. While using a new lens requires a bit of a learning curve, it is not as drastic a curve as learning a new camera on the fly. However, it might be a good idea to have a duplicate of your own camera in case something happens to yours on your once-in-a-lifetime trip. Nothing like getting there to find your one and only camera has pooped out on you! Renting a fancy camera that you are not familiar with will probably not result in your having great pictures. It will like result in frustration because you don't know your camera and can't get it to do what you want it to.
So, consider renting a lens on your next big trip. Is it cheap? Not really! But it's cheaper than buying a lens. You're spending money on your vacation - include lens rental in your vacation budget. Why not? You include meals, lodging, car, etc. What's important to you? If you are a photographer, you WANT great pictures from your travels! Also, renting lenses is a good way to decide if that lens is something you want to add to your gear. Try it out first! It's an investment - make sure you want it bad enough to drop that money!
Last minute tips: Don't forget to get quick-release plates if your rent a long lens. If you don't have one, you will be unable to mount it on your tripod unless you have another long lens that mounts with that little handle-like thingy on the bottom of the lens. Regular lenses don't need plates - they are mounted directly on a camera that is mounted via the plate to your tripod. Remember to include a polarizer for any lens you rent, as well. Also, make sure, before you rent a long lens, that your tripod will support that extra weight. You can't mount a 500 mm lens on a flimsy tripod!!! Do NOT carry your camera with the lens attached on your tripod. You can trip and fall and break lots of things - you and/or your camera and lens. Be sure to tighten all the "controls" on your tripod so you don't get FLS (floppy lens syndrome) where your camera flops downwards and makes your tripod fall over.....not a good thing! Also, don't pack your gear with the lens attached to your camera. You don't want to have your camera and lens damaged if something happens.
Most importantly, whatever your gear, have FUN on your trip! THAT'S what it's all about (and getting good pictures.....)!