Sounds like you will be having a lovely summer 
Just to be very practical, some things to think about before the holiday:-
Get plenty of poo bags!!
If she's a typical terrier, she may well like digging and exploring, so make sure your garden is secure and keep an eye on her digging a little tunnel (she may not need to drop sand down her trouser legs though! 
)
Find out from her owner who her vet is, and make sure their emergency number is in your mobile phone. It's also sensible to have her owner's details, and the dog warden programmed in.
Sort out with your neighbour what happens if woof needs vet care or any ongoing medication while with you - is she insured? Do you need the insurance details? Who will pay upfront? In an emergency, who could authorise PTS?
Find out when she was last wormed, and if she is likely to need any worming/flea preventive treatments while with you.
Get a little tag made for her collar with your contact details on. You can get these at Pets at Home or many hardware stores.
Some microchip companies allow you to put in your address while she is away on holiday - check with owner if this is possible. Again this will speed things up if she should ever become lost or stray.
Ask owner to bring one of her blankets, and some toys to help her settle in, and stick to the same type of food/time of feeding to help her settle. It's great that you are going to be walking her before she comes to stay, so she can get to know you, and vice versa.
I wouldn't recommend going offlead for at least ten days until you feel very much more confident about her bond with you. If you can find somewhere safe and secure, you can start by gently testing her recall - if good, then gradually go from there. If in any doubt, don't let her off, she can still have lots and lots of fun on walks, and you can get very long leads too, that will allow her to have lots of fun while out.
HINT - During the holidays, the charity dog show season is in full swing. Why not take her along, with DS, and do a few classes? Usually only cost £1 per class, so it's a very cheap afternoon out, and particularly for children they are practically guaranteed a rosette. Charity dog shows are great fun, raise funds for local rescues, and have classes like Sausage eating competitions, waggiest tail, best oldie, best child handler etc. Just do a quick google and I guarantee (weather permitting!!) you'll have a terrific afternoon out.
And don't forget to take lots and lots of lovely pics!!