Honestly this depends on their personality/interests. What might be an ideal gift to one person (such as a Diamond ring or other precious stone) may not be all that great for someone else. (Personally I'm the kind of girl who doesn't care much for jewelry, flowers, or makeup) What does your significant other use the most? Where do their hobbies/interests lie? Are they outdoorsy?
Chances are they've also probably dropped a few hints here and there to that "special something" they've been eying for a while now