Many of us who keep fish bought them under the idea that they are considered relatively low-maintenance pets, compared to say, a dog. That being said, though, we can get attached to them, whether it is a rare marine eel that cost hundreds of dollars, or a humble goldfish that cost a quarter... we still wouldn't want them to suffer while we are away on vacation!
1) If you are going away for just 3-4 days or less, DON'T WORRY, you don't need to do much at all. Fish are just fine without feedings for this long... some fish, in Nature, go through tough times when they don't get much to eat, so it is not cruel. Most healthy fish will be just fine in this short length of time if there are no other water problems. True starvation happens in weeks, not days.
2) Overfeeding is a far greater problem than underfeeding. This is why many well-meaning pet sitters, who may just be trying to help (but have never kept fish and don't know how to feed them) end up "killing them with kindness". It's not so much the feeding itself as the pollution due to uneaten food that can become a problem... this accumulates and builds up ammonia, a toxic by-product of any waste accumulation in the tank (normally an established bio-filter will convert this slowly to nitrite and then nitrate, but if it happens all at once, it overwhelms the biofiltration and can have deadly results on fish, especially in a tank smaller than 20-30 gallons). If you decide to have a pet sitter, the best kind is one that has a lot of experience with fish already. They do not need to come in every day... every other day usually works out well. And when they do come in, be very clear about how much and how often you want them to feed. If they have had no experience, physically show them what a portion feeding looks like, and draw out a concise schedule of feeding to make things less confusing.
3) "Feeding blocks" (sold as "vacation feeders" and under other names) are not always the best products out there, in my opinion. These are usually white blocks or sticks of food that dissolve slowly, releasing impregnated food. They are of some limited use, but usually only last a few days anyway (at most a week), and sometimes will pollute the tank if the fish are not interested. They are best used in larger tanks if you do go with one; but again, if you are only gone for a few days, sometimes it is best just to not feed at all.
4) Electronic or digital fish-feeders are in my opinion, a much better option. These will automatically dispense food on a set schedule that you determine (or for some simpler models, preset for once or twice a day). They are either battery-powered or plug-in models, and some even come with an air hose attachment to prevent condensation and caking of food. For greatest peace-of-mind, it's best to set the feeders and run them for a few days prior to going away. That way you can monitor that it is working properly and dispensing the amount of food you want.
5) A 25% water change a day or two before leaving is a very good idea. Regular percentage water changes every week or every other week always help the health of the system, so long as they are not causing drastic changes (that's why it's best never to do huge ones of over 30-40%). Water changes take away slowly accumulating wastes (DOC's or dissolved organic compounds, ammonia, nitrite, nitrate, phosphates, etc.) which stress fish, predispose them to disease, and also promote excess algal growth.
6) Buy a $2-5 automatic timer for the lights. These are super-cheap, and in my opinion, a great idea for even regular operation of the tank when you are home, because they save you the energy of having to turn lights on and off at a certain time. They are almost indispensable for planted FW aquariums, and SW reef aquariums, both of which depend on photosynthesis (a regular schedule of lighting will help keep conditions consistent). For fish-only tanks, this is far from a necessary piece of equipment, because fish will adjust to the dark, but I recommend them because they are so inexpensive. If you don't have one, it is a better idea to leave lights off than on, because it will create less stress in the long run (fish sleep too, they just don't have eyelids). They will adapt to the dark for a few days, but may be extra jittery when you get back and turn the lights on, that's all.
7) If you are very worried about power surges and power interruptions, there are battery-powered back-up air pumps you can purchase to keep circulation going in a tank where filtration isn't running. Keep in mind that airstones are not necessary in most tanks anyway that have proper water flow from filtration... most diffusion occurs through the surface, and fish don't need "bubbles" to survive. However, in a tank with no circulation, pollution can build up, so these products can help if power goes off. I believe there are also other similar back-up heater modules, but these are much harder to find. You may skip these types of optional products if you are only going away for a few days and/or are not worried about power interruption.
8) For the reef aquarium (SW aquarium with corals), going away for any time is a little bit trickier. Reef tanks usually cost several thousand dollars just to set up and corals can be extraordinarily sensitive to even small changes in water quality. Though there are some very automated and smoothly-running setups, including top-off systems that inject trace elements (like the Nurce, for instance), calcium reactors and the like, I do strongly recommend that if you are going to be gone for more than a week or two, that you do have someone experienced with reef tanks come in and to at least look in on the maintenance of the system. For longer times of absence, there is just no replacement for human experience and response to unforeseen things... don't forget Murphy's Law!