Firstly you are an awesome friend.
I have a DD about the same age.
Things you.might want to think about.
Sleep: get the exact routine in writing from your friend including wind down lighting in room etc. and ideally some of the bedding - eg sleep suit, My DD has a comforter blanket and has a dummy for bed only.
We read Spot the dog flaps before bed.
How to handle night wakes (milk or water only, any tips)
Little home activities- I'd get a couple of new cheap things/activities like play Doh, a sticker book etc. Just increase you are in a pinch for something fun to do. If not needed it's a little present. You can get that somewhere like The works for a fiver.
Also get friend to pack some small faves (for my DD this would be stacking cups, perspex coloured paddles you look through 🤷♀️ and a wooden bus - beep beeeeep! 😅)
Clothing: ensure there are spares and warm outdoor stuff inc hat. (See below)
Outdoor activities: most toddlers need to have a run about. Identify nearest playground and make sure there's a bit for smaller kids - there usually is like 90% of the time.
TV - if case you need it....what are fave cartoons (it's generally bluey hey duggee or similar)
Snacks & water cup - cannot have enough of these. Bring a selection and water whenever you go out. I also offer if hungry between meals or if she is a bit crabby
Get friend to pack or have a sippy cup in advance
in our house Cheerios, fruit and oat bars, baby crisps, the classic Na-na, berries and other fruits are all BIG hits.
Expectations: expect the emotion rainbow and know it's not personal and not that you do anything "wrong." My DD goes from whinging to giggling in 0.2 secs and can have an absolute meltdown over precisely nothing before being engrossed in stained glass / a sieve etc in same time span.
This happens min. 20 x per day 😅
Keep your voice nice and level and just explain what's happening or whatever nice and calmly. In desperate moments snacks may be offered 😆