If you are traveling light, you can use compact bags with a volume of 5-15 liters under the saddle, inside the frame and on the steering wheel. If the trip is long, a reliable trunk that can withstand loads of up to 20-25 kg will be an ideal choice. If your bike does not have special holes for mounting this, there are models with mounts for the seat post. They are installed on any bike, but keep the load less.
For luggage carriers there is a wide range of bags of various sizes and designs, some of which are protected from rain. Having secured the bags from the sides, you can safely place part of the luggage on top of the trunk, additionally securing with elastic bands. Do not forget that you need to load side bags evenly, and pack things that will be needed less often, pack deeper. Things intended for sleeping (sleeping bag, replaceable clothes) should be additionally packed in sealed bags or plastic bags.
It's time to talk about clothes. The choice of clothes is very important, since during the ride it is necessary to provide ventilation and moisture, and during parking - protection from the cold. You should not count on weather forecasts on long trips either and rain protection should be taken anyway.
The ideal choice is specialized bike equipment. Its cut is adapted for use while riding, and the weight and volume are very small. However, if a wallet does not allow you to purchase a full set of clothes for rare trips, you can use analogues designed for hiking, trekking or running.
On long trips, it is very important to ensure buttock comfort with long contact with the saddle. To do this, use shorts and tights with a special soft lining - the so-called “diaper”. If you are more comfortable in casual trousers than in bicycle tights, you can use the lower shorts paired with any pair of pants.
To divert sweat, we need good thermal underwear for cycling, but you can use more inexpensive analogues for running and football. Their shape is not so well adapted for cycling, but moisture removal is also at a high level. In the offseason, it is worth considering insulated jumpers for road cycling.
To protect against rain and wind, raincoats are suitable. They are both ultralight, and more dense and inexpensive. If you already have a good membrane jacket, you can use it, but in weight it will usually be inferior. To protect your feet from water and cold, there are special bike shoes that are designed for contact bike shoes, but some models can be used with ordinary shoes. Do not forget about multifunctional bandages - they will help protect your neck from blowing, and in the heat they can be worn on the head under a helmet so that sweat does not get into your eyes.
Additional comfort to the palms and a firm grip will provide quality Cycling Gloves - an indispensable thing on the first trip, when your hands are not used to being on the steering wheel for 6-8 hours a day.
If at night the temperature drops to 0 degrees, for warming in parking lots it makes sense to take a compact trekking or climbing jacket with you, because after intense loads the body cools much faster. They are very compact and weigh about 300-500 grams. In the summer, you can get by with a good hiking hoodie.