A cheap greenhouse bar heater connected to a relay controlled by a Pigrow, i've had this box (bluebox) set up the same for a few years but not in use for the last months - just turned it back on and happy to see it's still working as intended.
Did notice because the OS on the Raspberry Pi is so old that it's using a prior version of python that isn't compatible with a few of the scripts used by the new gui - it should be fairly easy to update it without affecting anything else, though if not i'll copy the config files and install a fresh Raspian (RasberryPi OS) then upload the config files back in place. No rush though it's doing it's job well.
Here's a video of it https://www.youtube.com/watch?v=jHNsRzzsbZ4
The video talks about controlling the fans but the heater is essentially the same, if you just want to log data and see the graphs then all you need is a Pi Zero (or any pi model) connected to a BME280 (a few other temp/humid sensors are supported but the bme is cheap and reliable)
I use the Pigrow software i wrote (free and open source) which can do most the complex stuff for you and make a whole range of graphs to break it down in useful ways. If you want to try it then i'd be more than happy to help you through setting it up, and there are guides on the github wiki and a few videos with more coming soon on the youtube channel.
If you do put in a Pi then also consider getting a camera (picam or webcam), pigrow can help you capture and assemble timelapse video too, it's a great way of seeing your plants grow and react to events especially when paired with the graphs (you can overlay them so they sync up) - there's a whole load of other sensors supported too and options for setting up watering schedules and stuff but yeah a pi and a bme280 is all you need to measure temp and humidy.