If I have to do one thing to monitor a simple website I'm probably going to use something that takes a screenshot periodically and checks it for changes. There are open source solutions but I just prefer to pay a bit for a managed service to do it.

I think it covers quite a lot of things - the servers are up, DNS is OK, assets are OK. It can also be a safety net in case of other, more sophisticated monitoring fails to detect an unusual state.

This doesn't work well for website with too much javascript, ads or widgets.