If it is your first pair: adjustable, and looser than feels like anything at all. You can always tighten. A fixed tension pair that turns out to be too strong gets worn once and never again.
If you want the sensation to build on its own: weight. Bells, chains and beads add a steady pull that keeps working without anybody doing anything.
If you want sound: bells. They turn every movement into noise, which does something entirely separate from what the clamps themselves are doing.
If it is a set piece as much as a toy: feathers, pearls and crystal. These are built to be seen, and that is a legitimate reason to pick one over another.
On safety: fifteen to twenty minutes maximum, and release slowly. Longer restricts blood flow past the point of being sensation. Take them off one at a time and give it a second, because coming off is the intense part.
