Thinking a bit along the Nest(TM) line:
- Simple on-board UI with e-ink display and a knob
- Remote access via web browser to change the schedule, etc
- Powered by a USB charger
- Battery backup when AC power missing (low-power mode keeps schedule and on-board UI alive but no remote access)
- No cloud services, registration or black boxes with Chinese firmware
Hardware?
- ATMega family
- Separate WiFi module which can be completely powered down
- Remote sensors using NRF24L01+ on Arduino:
- NRF24L01P spec
- tutorial
- enchanced library but for Arduino only
- simple library for AVR. This looks promising.
For stand-alone hardware, the Multi-Tens project is a possible starting point. Fits in a nice bud box. (wiki)