We do these as well, as part of the larger contract for the build cleaning of the new properties.
Usually, for us it's 2 offices, 2 toilets, a dry room and the canteen. And they are of the 'container' type.
I had to laugh when you said they are quite modern inside. I am assuming they have recently arrived from the suppliers. Wait till you are trying to keep them that way after a couple of months!
As DG has said, they are usually quite laid back because they see you struggling and sweating to mop a floor - then they walk past you and straight over it with their muddy boots!
But you must not let it get to you. You will be fighting a losing battle with the mud and dust. When (IF) someone says sorry as they stomp on your just mopped floor, your reply should be "That's OK, it was clean" In otherwords you have done your job. You won't go over it again. If you do, you will be there all day!
You are lucky to be getting a clean every day. You might be able to keep on top of it - which will make it a lot easier. We usually only go in once a week, so you can imagine... But the site labourer does have to sweep them now and then.
Just clean them like you would any domestic property or ordinary office: If it's dirty - clean it. Apart from that just the usual, empty bins, sanitise toilets, wipe down top surfaces and where they have kicked things with their boots, wash up crockery if that is a requirement and wipe down white goods etc.
We don't charge much for them, even though they are hard work, because often it is a way to get on the site early and get to know the site agents. After all, it is the rest of the contract that we are really after!
You will have your good and bad days (rain for muddy floors - sunshine for dust everywhere - you can't win!)
Good luck...