We will be taking our 4th back to back trip later this summer, having traveled in cooler climates and much warmer (85-90 deg) cimates on the same one month trip. Having to be in varying cooler and warmer climates on the same trip means watching weight of suitcase, too. Most airlines allow 50 lbs leaving the US and return (50 lbs is about 22.7 kilo). All of our back to back trips required a one-three days in a hotel between ending one trip and picking up the other, whether it was cruise, land, or combination of both. This hotel stay is at our expense, but I usually have GCt book it at the same hotel as the meeting place of the second tirp, so we don't have to pack our suitcase at one hotel then travel to another hotel to meet our other group. The flight to that next city is also at our expense, but I have GCt book that too. Never been disappoiinted either. We don't usually go to another city/country to sightsee before we meet up with the second group. We've lived in Europe 8 years and Asia 13 years, so we are content to just go to a hotel to do laundry, get more ATM cash, sleep in, chill out and walk around the area, or take cab to somewhere we want to see, get some supplies we may need.
By the way, if GCt books your between tours flight and hotel, you are covered with their travel protection plan; may have to pay extra for that, but not much. I don't travel without that protection. Also, you will have to pay for transfers to hotel unless you are meeting your river cruise on the same day, then travel is with GCT.
We usually pack for the trip that has the most consistent weather, ie, if most of the weather on one trip will be in the high 60's to mid 70's, and it is expected to be mostly partly coudy and rain, we carry 2 long sleeve lightweight sweaters,2-3 long sleeve jerseys, a water resistant windbreaker with hood and deep pockets, rain bonnet or rain resistant hat for me (woman) and baseball cap for my husband. Umbrellas are a must, but in a crowd, that is listening to the tour guide while stopped, the umbrella is not worth it to put up because they bump iinto another's umbrella. If the tour guide and travelers have the head phones, then you can use the umbrella and walk away a little. We always carry an extra pair of good walking shoes in case one pair gets wet. Gloves and a very light weight woven scarf if mornings will be in the 40's or 50's. A fleece vest is good, too, if mornings are that cold, with the sweater and windbreaker, you'll be warm then remove later in the day. Winter caps to cover ears also for colder weather.
For the warmer climates, we carry two or three short sleeve jerseys or shirts, maybe lighter weight pants, like the polyester/nylon ones that zip off at the knee for shorts for him. We found stretch jeans work well in both climates (cool and warm). This is because stretch jeans can be worn at least 3 times for men (mostly) and 4 times for women (because women can put some protection between the jeans and other garments to help keep jeans clean and they look good without pressing. Believe me, we've done a science on this. We alternate jeans so we don't wear them 2 days in a row. So, for 4 weeks, 3 pairs of jeans, maybe one pair lighter weight, or dockers. Stretch jeans as opposed to cotton denim because they dry faster, since we do not use dryers (our choice).
For sweaters, I found acyrlic/cotton or acrylic/polyester because they are lighter weight and dry much faster overnight. Same with jerseys, cotton/poly blend dry faster and are warmer. one camisole or layering jersey (like skiers) for under if you want. Be sure to carry sun hats or visors for sunny days. I also found crushed micro poly slacks for myself that are sort of crinkly, do not need ironing (not recommended), wash out and dry fast, and can wear cuddlers under for colder weather. So, good for all climates; saves on weight of jeans in the suitcase, looks dressier, too so I can wear with different tops for dinner. For laundry, we use the time in a hotel between trips to either get cleaned by hotel, or a laundry (leave and pick up later for a fee) nearby, or do by hand at the first hotel/ship or as we end the wear for that item on our first trip . Tops get cleaned more than jeans, for obvious reasons. If layering, the second and third layer does not have to be cleaned as often. We carry maybe 4-5 pairs of underwear, and wash out in the shower each night or morn. My husband just said today that he was goiing to look at what he takes because he always ends up taking clothes home that were never worn, especially socks and underwear, usually 1 pair of pants or a shirt. He has started buying diabetic socks and they seem to dry faster, but there are varying kinds of quick dry socks. On river cruises you can have staff do laundry, I find it expensive but convenient. That's why we do our own undies (bras or paties $1.95 or more). Things may have changed (hope so) if they charge say, $20, for a bag of laundry, any amount you can stuff in the bag. Good bargain we had on a Holland Amer. cruise. Would be good if GCT river cruises do this; I think would be more laundry business. $7.00 for washing shirt is too expensive.
For river cruises, he takes one or two pair of dress or docker type slacks and one or two sport or dress shirts; no sport coat. On GCt river cruises you don't have to dress up for dinner anymore and can wear just what you wear to tour in that day, including sport walking shoes. I, being female, carry one or two polyester lightweight dress pants with 2-3 tops; some that I'd wear to a nice restaurant in the US; my stuff is lightweight so I can carry more. I always pack the flattest pair of sandals I have (lightweight) that are sort of dressy to change into from the sport shoes for the evening. I carry a small purse very small/lightweight that has a long strap for dinner that I can carry face tissue and room key and I wear it sling.
When packing the suitcase, find out if your flight between two countries has differing baggage weight allowance because you may only be allowed 44 lbs (20 kg). This is happening to us this summer--between Paris and Athens, we are on an airline that only allows 44 lbs, but going and coming to Europe and back to the US we are allowed 50 lbs. So, I checked with the airlines that allows only 44 lbs and found that their poilicy is that we can pay for the extra weight and still be allowed to carry the 50 lbs--the 44 lbs are free, but the 6 extra pds per bag will cost us varying Euros depending on how I pay (on line while getting boarding pass, or at ticket counter). But, every airline is different, so check yours.
On our first back to back, I carried 8 paperback books because I like to read before I go to bed and often can't sleep, so read for an hour or two in the middle of night. Cut down on the weight of my clothes so I could carry these. The second back to back, I got an e-reader and downloaded about 15 books; I charged after I was finished for the evening; my husband's camera was already charged, so I could use one of his receptacles. e-reader chargers are usually 110-220V, so just need an adapter plug for the country you are in. Electrical adapters for Britain or UK are different than for Europe, so be sure to get for both countries.
Enjoy your trip.