If you don't need navigation and connectivity on your handlebars, the humble wireless bike computer that relies on a wheel magnet and sensor are still great. They're cheap, reliable, run for years on simple coin cell batteries and do all the basic things that are helpful to have on a bike without any fuss. Time of day, elapsed ride time, speed, and distance are all the vast majority of riders actually need.
I've had the chance to use fancy GPS computers from multiple brands over the past few years and they're very cool, but if you're already carrying a smartphone in your jersey pocket or saddlebag they're entirely unnecessary unless you're connecting to a power meter. Also, they require regular recharging and all involve some degree of faff to set up just how you want them. If I was building a cheap bike for someone else or starting from scratch myself I'd go with a cheap Cateye or something similar.
