If I had to pick one coin without knowing what the small object is I would probably lean towards a US dime or more likely the nickel (or a similar coin from another currency). The silver-like color works with more things than copper (penny) or "gold" (US $1 coin). And the nickel is thicker, which would help it stand out more in lower angle camera shots than the dime. Quarters can be a bit too big (ditto for the US $1 coin). An ancient coin would likely draw too much attention to itself (focus is supposed to be on the small object) and people would be less familiar with it, so it would be less useful to indicate size.
Problems with the others:
- Pen(cil) and Keyboard: Less flexibility than the coin. Implies certain settings (office, on a desk).
- Ruler: Little artistic value. Will likely draw attention from the small object. Looks out of place outside.
- Credit card: Everyone will be looking at the number.... ;-)
- CD and Floppy: Less flexible and may date the photo -- e.g. maybe use a DVD or BD today instead?
- Door key: One of the better choices, but still less flexible than the coin (key in grass or sand?)
- Thimble: One of the better choices, but may draw attention away from the small object and looks a bit out of place outdoors.
- Drinking glass: Too big.
- Human hand or finger: varies in size too much.
Anyway, the short version is that coins offer the most flexibility for generating a photo that is both functional and artistic.
Elda Taluta; Sarks Sark; Ark Arks