Really depends what you want to do. The Intel Edison kit from Sparkfun has an OLED screen with buttons on it. You can easily add sensors, speakers, etc. It run

Really depends what you want to do. The Intel Edison kit from Sparkfun has an OLED screen with buttons on it. You can easily add sensors, speakers, etc. It runs Linux and can act as an access point (but not at the same time you use it as a station) so you can ssh in from your phone and do more complex stuff. You'd need an enclosure of some kind but 3D printing is common these days. Another option would be something like the TinyDuino, which has a wearable kit. It's Arduino but maybe it could be made to work. The new Raspberry Pi Zero W would be the perfect form factor but I haven't seen any interesting in terms of wearables, and it runs at 5 volts which is a bit discouraging for this use case.