If you can, get yourself down to Kaikoura on the south island I absoloutely recommend the whale watching boat trips. You are near guaranteed to see Sperm whales. On my trip we also saw Dusky dolphins, seals, Orcas (killer whales)... and an albatros. Fantastic if you have an interest in wildlife.
http://www.whalewatch.co.nz/index.asp
Just seeing New Zealand is enough, it is an amazingly beautiful country and so sparsly populated. The south island is much better than the north from a sight seeing point of view.
There are so many things to see, I agree that you might find a £250 budget a little restrictive for seven days, even though NZ is not a particularly expensive country.
haha I keep adding bits...
Jet boating is a New Zealand speciality and is both great fun and a good way to see some scenery. It can be done at several locations. This is the one I went on, again in the south.
http://www.alpinejet.co.nz/