Here's the somewhat passive aggressive approach.
1. Make sure you either arrive at work before your boss each day, or leave after your boss each day, or at least the large majority of days.
2. A couple times a week answer an email or two during the evening or weekend, preferably ones your boss will see. This doesn't have to take very much time. Don't force it, make sure they are emails that you are actually interested in and have a good response to.
3. Work smart and hard when you are working, be above average on your team despite probably spending less time in the office.
4. Work longer hours than you normally do when it's something really fun and you don't have much going on at home anyway. That situation does arise from time to time. Make sure, in a very natural way, that your boss notices when you do this, probably by telling him/her how fun it was.
5. When you have to duck out for an hour or two for the dentist or whatever, don't tell your boss unless you absolutely have to (e.g., you have to miss a meeting with him/her). You are an adult, he/she doesn't need to know where you are at all times. You don't ask permission to go to the bathroom, do you? In short, don't emphasize to your boss, "I'm not going to be working for the next little while" if you don't have to.
6. Keep all commitments that you make to be at work at a certain time. Try not to make very many of those commitments. Instead commit to getting work done, not just being somewhere.
If you are doing all that, you can work less than 40 hours most weeks and nobody will care.