There are plenty of great restaurants in CWB. In fact, a lot of people go there just to eat! A few places that I like to go are Muji (Japanese deli, I guess; great when you want something a bit more healthy), Toast Box (Singaporean), Northern Dumpling Yuan, and Aladin's Mess (Indian; better for lunch).
There's a ton of different restaurants, though, so try looking around. Go on Openrice and search Causeway Bay restaurants. Not so sure on the bars, although I know there are some. I'd also recommend checking Openrice for that.
As far as cooking, I spent a long time with just a hotplate and a microwave. I actually got so used to cooking that way that I don't even use my oven that much anymore.
It is definitely WAY cheaper and healthier to cook for yourself, depending on what you are cooking (Western ingredients tend to be expensive). I did a lot of stir-fries. Get a rice cooker so you can cook rice while you cut up some veggies and meat and cook that in a pan. You can vary the meat, veggies, sauces and spices so the taste is different. Cous cous is also a good option because it can be prepared so quickly. Sometimes if I don't feel like too much i just cook some bok choy and cook a few eggs to put on top.
If possible, I'd recommend going to wet markets first to buy your veggies. It's much cheaper there. And if you're just cooking for one person, you probably want to buy just a few things, as opposed to a package. Not all vendors will just sell you one bell pepper or a handful of bok choy, but keep looking until you find one who will. Then go to the supermarket after that to pick up anything missing at the wet market (like if they are sold out or not good-looking).
I'd also avoid the international supermarkets except for specific things since they tend to be much higher priced. And when I go I'm also tempted to buy so much more expensive stuff because I'll see things from home that I like.
Anyway, good luck!