You call a wafer-scale BGA package that looks to be about 1mm square "easy to implement physically"? 
http://www.ti.com/lit/ml/mxbg091w/mxbg091w.pdf
I might consider the Maxim Integrated temperature sensors (e.g. DS18B20), there is good support for the Maxim/Dallas one-wire protocol. Another option is any number of I2C temperature sensors. Not knowing whether this sensor network is a board-level network or something covering square miles, it's difficult to make specific recommendations.