Will we have enough good weather for you to get a decent amount of use out of it? I think I would go with a trampoline over that. The people I know with trampolines get a lot of use out of it, and it keeps the children entertained and exercising , in view of the house.
Have you considered a giant play house/tree house if you can afford it, one that DC can play making house in when young, and sit about in it as a den house with friends when older, then a stop them from drinking and smoking in it hang out space for when a teenager. Yes you'll have to spend more to get the last item, but maybe get it as a 'real' birthday present?