I went with the Dome Fossil and Kabuto(ps), knowing that my cousin had already taken the Helix Fossil, and that we'd trade later on. I never really used it, and actually left it in the Day Care for quite a while just so it would level up while I was doing other things, and could evolve it for my Pokedex when I took it out.
I also forgot about the Old Amber for a long time, even after I'd long beaten the game. When I was trying to complete my Pokedex, I realized that it was one of about only 10 that I had left, and that's when it clued in I didn't have the Old Amber. I never ended up using Aerodactyl either, so it just sat in the box as soon as I got it back.