For a gift for my SO, I try and think like this:
1) Figure out roughly what I'd like to spend
2) Find something in that price bracket that they normally wouldn't spend that much money, but would like.
For $100, what does he like that he wouldn't spend a $100 on? Scotch or other whiskey, a luxe basic t-shirt, nice boxing gloves. These are just some things that I might like in that price range that I normally don't spend that much on.
Hope that's helpful, I usually have write a quick note on my phone when I have an idea for the gf throughout the year that way I'm prepped for christmas/birthdays.