homeassistant

If you like statistics give graphana a look. It's a very powerful graphing toolkit, given enough time you can create really beautiful views of your data.

If you need aditional data: Many smart plugs can measure power consumption...

Before buying more sensors/actuators: Think about the communication pritocols you want to use:

WiFi

• No additional HW required
• Simple setup
• No battery powered devices or high battery consumption

Bluetooth

• Probably no add. HW req.
• Battery powered devices
• limited range

Zigbee

• good range
• lots of devices
• low power/battery powered
• needs add. HW (usb stick)
• may be a little more complex to set up

(This is just a veeery short summary)

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.