If I had had more time the last time around, I would have build in a 'secret' vault that is controlled by RFID. I have an RFID chip in my hand and I'd be able to open the vault by waving my hand in front of it.
You need to think this through if you want to do it, though - devise a way to unlock it mechanically, make sure it doesn't open when the power is off, etc.
More mundane things:
- induction cooking plates in your kitchen. That's a no-brainer.
- Ethernet to a few places, but don't fuss about it. The jacks will never be where you need them, anyway. Instead, make sure you have a solid backplane so that you can put wifi repeaters on each floor.
- Floor heating, as mentioned above. No-brainer too.
- For optimal living comfort, thick insulation. 10cm (4") of PUR at the least, preferably 16 (6"). Make the house air tight. Put in a ventilation system, preferable with active intake and exhaust, with heat recuperation (it's called 'System D' here, don't know if that's international).
Also, put in an easter egg for the next person remodeling the house. Write a message on an inner wall that you drywall over, put a waterproof sealed envelope with some pictures of you and your family in a false ceiling, those type of things. It's awesome to discover one of those (I once did).