The answer to this depends to some degree on the nature of your job. Without knowing what your job is, it is hard to say for sure what you should do.
With some jobs, it is possible that there really is nothing left for you to do. If your job consists of making certain goods (let’s say, for instance, that you work in a woodworking shop), there might be nothing left for you to do once you have completed your projects. In that case, if you have made sure that all of your tools are cleaned and put away and your workspace is ready for the next day, you could justifiably do something for fun. You could read or you could use the computer, for example.
But most jobs are not like that. In most jobs, there are any number of things that you could be doing. At the very least, you could go through your old emails and delete ones that are not needed. You could make sure that all of your routine paperwork is up to date. You could clean your desk. If you are a good and motivated employee, you could get a start on whatever it is that you are likely to need to do tomorrow. In most jobs, employees have plenty of little tasks that could use doing but are not essential. These are the sorts of things that you should do if you have completed all the projects that absolutely had to be done today.