Been a while since I spent time at home-barista.com, but it's a good place to kill a couple of months while you learn in-depth about whatever type of coffee prep you prefer, and will lead to equipment purchases and some time fiddling around with said equipment while you train yourself on the usage.
If you like cars there are tons of forums. I would say search for one that aligns with the car(s) you have. Kill a bit of time reading all about the issues and fixes others have done on your make/ model, as well as mods and upgrades. Years ago I spent a bit of time on swedespeed.com, and still visit on occasion. There may be a local forum for whatever make/ model you have, which is nice because it can lead to more personal interaction, lending a hand on DIY projects, etc.
Garagejournal.com if you are a sadist. Zillions of posts, about any and every minute thing that might possible be in a garage. Massive, massive time-drain. I had to drop it after a couple of months.
Into cameras/ photography? Lots of forums for that. I spent a bit of time a while back on the Pentax one. The same people who run that forum also have a Nikon one. Posts cover all aspects of equipment, photography techniques, workflow, etc.
If you like watches there are a couple generic forums, like Timezone and watchuseek. Also many forums specific to brands.
There are forums for guitars
and cigars,
for screws that are loose,
and even for Dr. Seuss.
(I made up those last two, but it would not surprise me if they exist)
