In the "Glacier", seed, Walk toward the floating island away from the spawn point, then once you get near the water between the spawn and the floating island turn left to the small island in the lake between the mountains. Wade straight through the water to the other side facing a cliff with a giant hole/cave to the left and an opening between two mountains to a small wooded area then a desert to the right. However, in the middle, you will see a small cave in the cliff that appears to be a dud cave and when lit up, will appear a short tunnel with a gravel wall at the far wall. Dig up the gravel and- oh! What's that fiery cage glistening in the dark? Yep, you've found a dungeon. A spider one, too.
If you want to make a grinder out of this dungeon, the easiest thing to do is cover the opening into the dungeon with cobblestone, then make a hole in it just big enough for you to walk through-but not the spider. Then, lure a spider up to the cobblestone wall and start hitting it with you're bare hands, with the spider's defeat guaranteed. Collect the string and you know you've finished your manual grinder.
Post the dungeon locations you've found in different Minecraft seeds and ways to make grinders for them.