The most important thing about multi-day hikes is to have everything you need and a reliable source of navigation. Also, follow these two rules:
Always have a backup.
Always have an extra day in the hike in case someone is struggling, someone gets gastro or ideally so that you can enjoy somewhere that is particularly spectacular.
We have a two pronged approach to this, using your google calendar (or whichever calendar you use) along with a backup folder of screenshots on your phone in case the internet does not work. This is how we do it:
Create a calendar entry with what we are planning to do that day.
Add it as an attachment to the calendar entry, click on "Add a google drive attachment", then go to the upload tab and upload your itinerary or alternatively, put all your tickets in one google drive folder and find it there.
After uploading them to the calendar I go through the calendar entries on the phone, open it, screenshot the QR code and then make an album on the phone. This has two advantages, there is a backup on the phone that doesn't require internet and you check that everything is there.
Print out all the tickets and put them in a waterproof folder and bring them as well, you won't regret it.
As little as possible. Here is a guide to packing for multi day hikes. Get everything as lightweight as you can, the only things that you should take a bit more than you think you need are:
Water
Gas
As you can never have too many hot water bottles or hot chocolates on a hike, it's really criminal not to have enough of them though. And if you are an experienced hiker, you know how painful it is not having enough water. Take at least an extra 2L between you as well as each person's minimum, you can adjust this as you go, better to have a bit too much water than not enough though.
Here is a link to the best things we have found to have in a first aid kit, we recommend shopping for these items in your local pharmacy and storing them in a drybag with a first aid sign on it that is visible on the outside of your pack.
Here is a link to the remote first aid kit with a google doc that you can download it.
Here is what we use for navigation:
At least one backup phone and powerbank/ solar charger
On all the phones, the maps are downloaded on Avenza, Alltrails and Gaia
When hiking, keep your phone on airplane mode so that it conserves battery
Print out or purchase a topo map of the hike, to find this we'll often first find it on Avenza maps and that gives the name, which we'll search for online to buy. It is worth having a real map and compass as a failsafe and its a good momento after as well. If there is a guidebook it is definitely worth buying that instead and bringing it with you - if it shows you where one incredible thing is that you would not have otherwise found, it is definitely worth it.