My OH travels for work. He has a smart business travel laptop rucksack with specific laptop storage section and space for overnight clothes, pair of shoes etc which he loves as it means he can have both hands free when zipping through cities and airports OR he only has a small cabin sized bag to hold with as his hand luggage is on his back. It's similar to this amzn.eu/d/8Fmudhv
Photocopy of passport, driving license etc. We had a student who was able to fly back into the UK on a photocopy after loosing theirs.
EHIC (or whatever it is called nowadays)
Book accommodation through something like Hotels.com as you accrue a free nights stay after 10 bookings which he uses as a treat for us or his parents. Same with fueling up - look for a Tesco's and build up Clubcard points or a credit card with benefits like air miles or whatever is available.
Trains - he books first class on the way out so he has comfort, no fight for a seat at busy times, breakfast and drinks are provided and he gets wifi for working. He doesn't always bother for the way back. He won't use Grand Central trains as they often have issues. Book seats where possible.
Airport lounges (look for deals on sites like WOWcher etc) for the above reasons.
Some airports are busier / understaffed / security needs so going through passport control may take longer. I think he had a 60 or 90 min queue at Schipol, Amsterdam so next time he booked priority boarding as he had a connecting flight that he couldn't miss.
Look for accommodation with a gym or pool (if that's your thing) to make the most of your down time. When in the UK he prefers to stay in a nice pub if he can (Good Pub Guide is his Christmas present from the kids each year) rather than a faceless hotel. He may stay in a hotel but use the book to find a pace for a nice meal. He believes that as he is working away from home, he should have somewhere equal to or nicer than home.
He has developed an encyclopedic knowledge of service stations so will only stop at nice ones. He gets to claim a certain amount for an evening meal if he is home after 8pm so often stops and picks up something nice from M&S at a services which we both enjoy for tea the next night. You'll build up knowledge of places nicer than services to stop e.g. Mainsgill Farm which is 4 miles west on the A66 from Scotch Corner services on the A1.
He knows where he can get cheaper fuel close to motorway junctions, rather than filling up at services, like at a supermarket which is close to a motorway junction. This came from using his old Tom Tom sat nav which had a find fuel function. This is also helpful for when you realise you've left your shirt at home and need to pick something up quickly.
He is clever with appointment bookings too. Say he had a meeting in Carlisle, he might book it for the Friday or Monday and we'd have a weekend in the Lakes and his company will have paid for some of the fuel.
Airpods have revolutionised his work calls apparently by blocking out external noise.
Shoe buffer. Multi-plug. Powerbank. Good book. Snacks / cereal bars. Mini toiletries. Ear plugs.
He always asks for an iron and irons his clothes at the hotel so he doesn't need to worry about keeping next days clothes crease free when travelling. Just ask at Reception when you check in.
Do your expenses asap and get into that routine of doing them frequently. Sometimes my OH misplaces receipts so its harder to claim the money back. Take a photo and have a special envelop or clear pencil case to safely store them away. His work have a policy where they need claiming within 6 months or they wont pay them. Read your expenses policy and speak to colleagues about what you can and can't do.
He has a plastic box in the car where he keeps most of his regular equipment needed for his job. He just lifts it out and leaves it in the house when not away.