You can walk to Causeway Bay from Happy Valley. We stayed there when we first arrived, and I did it repeatedly. However, you can also take the tram, or a bus, or even a minibus.
The downside to Causeway Bay is that you would effectively be living IN the middle of a business district. If you intend to buy a car at some point, it will be hard to find an apartment with parking in Causeway Bay. Having said that, if you want to be above the action of cars and people then it is the place to be.
I like Happy Valley because it has a nice mix of western/chinese shops and people. You can get 'larger' apartments in Happy Valley... but unless you are willing to spend $$ or move out to somewhere like new territories, remember that space is at a premium on HK Island.
There are also a couple of good outdoor playgrounds plus the (free) indoor playground in Happy Valley. Everyone I've met who lives in Happy Valley seem to enjoy their location (unless they are in a steep area).
One thing to keep in mind when apartment hunting is kitchens can be quite different here. If the numerous places we looked at, only two had an oven... so if you like cooking, you may want to keep than in mind.
I live up in Jardine's Lookout, and while it is green, I feel somewhat isolated because to do something trivial like go to the grocery store I have to either climb a steep hill (which would be fine if i didn't have 2 kids, a stroller, and in summer you had to contend with the heat and humidity), catch a minibus to Causeway Bay (again an interesting challenge with a stroller) or drive.
Your serviced apartment SHOULD come with a washing machine, but dryer is not as common. As was said, most people hang their washing. When you get your apartment, if it has no washer, you can get a washer/dryer combined machine, but do your research as not all of them work well. We have a whirlpool one which is pretty good.
Good luck with your move, and once you're ready, feel free to join us for a mothers group meeting in Happy Valley sometime.
