OK, we pretty much always use http://www.holidayhomes.co.nz/ or http://www.holidayhouses.co.nz/ for accommodation. We ALWAYS pay extra money for a cleaner at the end of the stay so we are not stressed out on the last day. Remember that some will have a linen option, so if you don't want to bring all your own sheets, towels, tea towels etc make sure you find one that offers that.
Akaroa is awesome for : boating, nature tours (on the boats, Canterbury Cat etc) which I think are quite affordable, looking in shops (not a lot, but still not bad for its size) and awesome wee drvies to the other bays and beach exploring (Le Bons, Okains etc), and free fishing off the wharf. There is a nice restuarant for you guys (if you can manage with two little ones - Harbour 71) and you can find a resonable coffee.
Hanmer: personally not a favourite of mine, but that's cos I am a 'coast' person not a mountain person. Awesome hot pools (but pricey), nice cafes and resturants, shopping getting better. Good nature walks, a good golf course.
Kaikoura is great for: free coastal exploring - rock pools and seal colonies. Good cafes, resturants and fish and chips! Free fishing off the wharf. Interesting shops to browse (although again, not many of them), close to lots of easy walks. Great info centre with heaps of ideas.
There's also other places, if you are more a skiing/mountain type person like Methven, Geraldine etc.
Also some small towns can be interesting - try Timaru or Oamaru - interesting small towns with interesting waterfronts etc. I do find that the holiday houses websites have less in these places, so you might need to look at a motel. If you go as far as Oamaru then Moeraki is totally worth a look, and not just the bolders!!
Other really small, but amazing places (often no shops) close to Chch are Gore Bay, Port Levy, some of the lakes around Lake Sumner....
(can you tell I love NZ travel!!???)
Good luck deciding!