IP and User Agent, for example. Goaccess[1], a tool to generate statistics from webserver logs, is capable of calculating unique users. Calculating unique views entirely on your own server without any of that data leaving it, is way more privacy friendly than urging your users into accepting cookies so that Google can harvest their data and send it to their US servers.

I wrote "disrespecting" because using GA is exactly this for me. Website owners give a f** about your user privacy just to save them some work, without caring about any of your users' data.

[1]: https://goaccess.io