Helpful hints from a recent returner
Water shoes- The closed type are better than the open type. Close to half of your landings are wet landings - some on sand and some on rock. Some people took the open/sandal off and walked barefoot but that wasn't always a good idea. For those that wore the open style they pick up all sorts of rocks/sand and, even after cleaning them off once, pick up sand again. Something like the TEVA Churn is a better style for keeping rocks out and you just wear them wet (With socks is suggested). The ship takes the wet shoes at night and brings them to the engine area where they dry for the next day.
Snorkel equipment- They have a good selection of equipment including shorty wetsuits that we wore most of the time. Plan to wear socks or some protection in the flippers as they tend to get "hot spots" where you toes stick out and the top of your foot. You often have a "deep snorkle" in the morning and an optional beach snorkel in the PM. You see good things on both. Please don't come planning not to snorkel- you are just missing too much!
Underwater camera/zoom camera- Half of the trip creatures are down under and the quality of video is pretty good with most any underwater camera. Three people had Go-pros - but I had a Fuji-Film XP-70 (Now XP 80) that was only about $150 and it did just as well. I also had a cheap $10 "selfie-stick" to extend my reach and bought an special 18mm wide angle lens ($50) that was great for both Machu Picchu close-in shots and underwater work (It has an underwater mode as well as several others - panoramic, burst, etc). Being waterproof - you could carry it in the rain/wet landings without concern. I would suggest at least 2 extra batteries. With pics and video, I used up just a little over one 32 gig card. Make sure your SD card is a class 10- not class 4 - better for video. Finally- I would suggest at least one additional camera with a 20-30X zoom (The Fuji only had 5x). You can get a good Canon that has that and it's not as big and heavy as those larger SLR types.
Dry clothes - While on the ship they have a dryer but, with 19 other cabins, you have to find time when it's open. With the hot days and multiple snorkeling - you can go through a lot of clothes that don't dry quickly in the room. A couple things that help - bring some (6+) collapsible hangers to hang items from the vent itself as there are few other places to put them where they aren't against the wall= slow drying. Also, put the room vent/air conditioning fan on high - it helps circulate.
Wasps - One island has a little wasp problem if you wear bright color clothes (Orange/red/pink). Be sure to have some green/blue/grey options. Some people had to buy a shirt. Even little accent colors (backpacks/bottle holders) will attract them. Our group was ok, but the wasps were definitely around.
Walking sticks - Our group bought a lot of them- the typical collapsible metal kind that can be brought in luggage. They are as cheap here (<$10) as at home so you could just get it here. If you haven't used on, be sure someone shows you how to set it up - There are three sections (Some thought there were only two) and some people extended the poles too far where they would collapse if leaned on. If you see the word "stop" - You've extended it too far. If you bring some from home, be sure they have the rubber tips on it. They are required in Muchu Picchu and Galapagos.
Lima extra day - We got there a day early because we took a red eye. We just took a 3-4 mile walk through Miraflores (Where the hotel is) to the "Indian market" and through the park along the cliffs. It is a safe area and you can see a "cat park (Kennedy Park)," para-sailing, etc. Others took an official tour of Lima and liked it. The hotel will help with either.
Power- Not only do you want a converter, but you also need a multi-plug adapter so you can plug in more than one things as they often only have one 110 plug if they have one at all. I also got a multi-USB plug which powered up my camera/pad/phone/walkie talkies - all at the same time. Walkie talkies - It was a backup plan if we ever got separated where cell service was lacking. Never used them- but it was just a $24 investment and had a range of 1/2 mile.
After Machu Picchu you only have one hotel stay that is more than one night (Cusco). Most took advantage of getting clothes clean while there. The hotel charges $6/kilo but the guide had a person that did it for <1/2 that price. Have a bag to put your clothes in or you can just use the Tauck bag. If you are super sensitive to smell- bring your own fragrance-free detergent, but for most, it was fine.
Notebook/I-pads/computers - If you like to track the events, bring a couple small notebooks for writing notes. Many people brought pads/computers and you could get WiFi at most hotels, but the ship only had it in the lounge/sundeck areas and it was sssslllllooowwww! No sending large pics/skype/etc. Cell service was non-existent on most of the Galapagos portion.
Rain gear - We had raincoats, ponchos, umbrellas, backpack covers, etc. We didn't get a lot of rain but when we did, the ponchos ended up being the easiest to work with - covering backpacks, better ventilation, etc. Again - having the water camera also helps if rain threatens where all the SLR people were spending all their time juggling umbrellas. One other thing - head gear - not all comes waterproof but you could get a spray-on water-proofing that will help - putting that on before you leave.
Altitude sickness - Wife used prescription meds (Acetazlamide) and I used something a skier friend used (Oxygen Max). I had problems, she didn't. I think I didn't cut back on my eating (They say metabolism slows down) and that was my problem which left me fairly ill for just one day and then I was ok. Neither of us had any alcohol for the first couple days. Guide also seem to think the fact I am a runner (~5 miles a day) that he has found that runners have more problems- which I found odd. I did find I felt find when moving, it was when I was sitting still (I have a runners low heart rate) that I felt the need to take deep breathes.
Suitcase size/luggage issues- We followed all the rules and size and weight and in the end, if you were below 50lbs checked and 20lbs carry-on, they didn't seem to worry about size. Leave room for all the goodies you are bringing home. One couple had their bags get drenched at the Miami stop-over - consider hard-sided or putting a lot of plastic around your stuff on the inside/outside. TSA locks - Only to be used for the US flights, not for all the Tauck tour flights. When leaving Ecuador, plan on a special bag search of some sort. They will call your name and take you to a area underneath the concourse and have you pick out your bag and then they will go through it. Not a big deal if you aren't doing anything wrong, but get it done when they call you (They may really botch your name so listen for it) or they hold you from boarding and you might miss your flight.
Endurance - None of the walks was more than a mile and only a couple that went up any sort of hills. Go for it if you can, the views are great.
Overall - Good accommodations/food and even though they downplay the ship as an expedition ship, we thought the room and food was good there as well. This was my wife's dream trip, not mine. But I found I really enjoyed it and especially the wildlife in the Galapagos. We had both good guides and a group that was on time and had no divas to deal with- just good people interesting in learning and experiencing all there was. Have fun!!!!!