A few water bottles, preferably you should have two on a cage at once. Just in case one empties/breaks or if they're pissed off at each other and stopping the other getting to the water. It's not common but these things can happen.
Two or three ceramic food dishes, just so you can throw one in the dishwasher/have a spare in case you drop one/etc. A larger, flatter plate-like dish is good for fresh food too. Ours get some of our dinner most nights and/or fresh fruit and veg. Sometimes tuna or a good quality pure meat/veg cat food (Applaws do pouches of chicken and veg which can do two or three days worth easily). Though the extra protein is only once or twice a week. These are good all-rounder bowls:
www.ratrations.com/rat-dish-snugglesafe-p-913.html
Pet shops have them too but they're a good size.
Hibiscrub is a good thing to have in the cupboard. Good for both human and animal wounds as it's a disinfectant.
Make sure any tetanus shots are up to date. You actually don't need them for fancy rats but if you get a bite and see a doctor for reassurance it's the first thing that they ask! I was sent to A&E on a Sunday by NHS Direct (or whatever they are now) a few weeks ago because I was a twat pissed off a Mummy rat by taking her 2wo babies and she bit me. After a prolonged visit at A&E the nurses I saw both said that as we'd hibiscrubbed it and covered it they weren't too worried and sent me home.
Some wooden rodent chew toys.
A hanging wooden parrot toy or two (good for chewing and climbing on)
Lava ledges
Cardboard boxes, any size and shape, they love using them as hidey holes
Cereal boxes with one side cut off make brilliant litter trays for bigger rats, our boys stopped using the large trays we bought but do use a cereal box (and it can just be binned, win-win!)
Nuts in shells, walnuts are particularly great. They're brilliant toys. You can buy bags from www.ratrations.com/
Rope toys, ours love them
Bird ledges/perches are usually good, the silicone ones rather than ones that are harsh on the feet.
Baby rings for hanging hammocks
Biscrok milk bones. Rat crack, I break mine into little pieces and use them to train Ashi.
Yoggies, ones with as little crap in as possible. They're good occasional treats.
Dried fruit is another good occasional treat
I've recently bought these little stick things, they're multi coloured and look like little pretzel sticks but at all different colours. No clue what they are but they're also ratty crack.
Peanut butter - handy to have as it's good for ill rats (fattens them up), brilliant for medicating them (mix meds in to hide taste) and a nice treat for them too. But it's a choking hazard so always mix it with a little water and/or honey on a plate before using it.
Be careful on nuts/seeds, they're very fattening so if they're in your mix as well just don't go overboard on treating them with them.
Here is a decent list of things rats shouldn't eat. Though ignore the bottom, you won't find lab blocks in the UK and most people use a meusli style mix. They generally can have most things so it's worth printing a list and sticking it somewhere that you'll see it:
www.ratforum.com/showthread.php?39486-Good-and-Bad-Food-for-Rats-amp-Ratty-Recipes
The main ones that are commonly in the house are poppy seeds and apple seeds, both can be very harmful to a rat.
I don't know if I linked you to it but this is one of the common mixes people make for their rats:
www.fancyratsforum.co.uk/viewtopic.php?f=14&t=56
It's called the Shunamite, that's the updated and better version and it's the only place you can find that version. I'm not sure why she hasn't updated her website. If you can get a copy of The Scuttling Gourmet it's a really good book about diet for rats. I am struggling to get a copy but apparently the NFRS sells them. I've borrowed my breeders copy for now!
Find a vet now who knows about rats, though IIRC you have a relative who is a vet? Or was that somebody else?
If you're doing a shop at ikea any time soon, they have so many items online and in store which are brilliant for rats.
This fleece is brilliant, and no stupid seams on it:
www.ikea.com/gb/en/catalog/products/70242821/
We bulk buy it as we use it for makeshift hammocks, I sew hammocks with it too, it gets used on the bottom of our cages as we have aviaries that don't hold substrate. It also gets thrown on the sofa to cover it just in case of accidents.
This is really interesting either hung up or horizontally making another layer:
www.ikea.com/gb/en/catalog/products/70108912/
I've seen people using these but I can imagine that they have to be binned eventually:
www.ikea.com/gb/en/catalog/products/40123457/
Anything can become a rat toy