For gadgets and tech stuff in HK, definitely try to hit up Mongkok.
I really wouldn't know where/what else to see in HK. Ocean Park, maybe? I usually only hit up HK with family to visit family, so I rarely care to go out on my own.
If you have some time in Tokyo, hit up the Shibuya area or Odaiba island. Shibuya is a very cool and hip area with lots of young fashion and most recent trends. Odaiba's the man-made island that once had the 1:1 scale Gundam, but I think it's been moved elsewhere. Plenty of cool shops and structures there; there's a pretty decent TRU there as well.
If you're looking for specific places to hit up for toys in Akiba:
- Yodobashi Camera (on the east side of the Akihabara JR train station) - for all your new/retail releases
- Mandarake - floors 7-10 (if I remember correctly) is all toys of all eras, new and used
Those are the 2 big places I'd recommend. There are obviously plenty of smaller shops that will have hidden gems, but I wouldn't know them off the top of my head so you'll have to discover them on your own.