Mini D1 board with 3pin connector for Dallas DS18B20 temperature ssensor. Remeber to put API and filed number on a sticker.
WORK IN PROGRESS
This might be the easiest and cheapest way to make an internet connected temperature sensor. It requires some basic knowlegde about Arduino IDE and using some tools.
Soldering Iron, snips, usb cables, ESP8266 development board or 3.3V USB TTL interface, some solid wire cables, shrinkwrap, batterycase
Find cheap and easy to use ESP8266 modules on ebay or aliexpress. About 2$.
A bunch of DS18B20 from ebay