I've really cut back on knife collecting, only buying a couple a year, mostly from folks on the forum. My other hobbies are photography and rock collecting/polishing. For photography I'm straight Canon, never sell anything so I have four DSLR bodies from 40D to 5D IV, lots of lenses, too.
Rock collecting is great, we're outdoors doing something fun. I, also, purchase rocks and tumble them to a full polish. Never sell any, I give away to young people that show an interest. I finish 8-12 lbs a month, adds up fast!
I have a few mini-collections - carved birds, Tim Cotterill frogs, Utah guide books, ghost town books etc.