I had a cushy job like that, could easily do a days work in a few hours and remain ahead of my colleagues. Then they put a load of tracking software on our devices so they could see what we were doing which put an end to it.
I hate the idea of being in the office just to be seen to be doing something. I much prefer being given my allocation of work and cracking on with it, if I can get through it quickly to the standard asked for then what's the problem.
The trick like you say is to meet targets but not go to far over the top. If you keep exceeding targets then they just give you more and more work. Plus your colleagues won't appreciate it, we never discussed it but I was pretty sure they were doing the same.
I left in the end anyway as the work was so dull.