If you want something inexpensive to play with, start looking at Zigbee devices. There are a variety of environmental sensors, plus motion sensors, contact sensors, and smart lights, sockets and switches.
You will need a Zigbee coordinator to get started, popular choices include the Home Assistsnt SkyConnect, SONOFF USB dongles, and Conbee II.
Building custom devices with ESP8266/ESP32 boards and ESPHome is also a lot of fun, there are plenty of projects for weather and air quality monitoring for instance.