Sebastian,
In Canada, while 50 cent pieces do not circulate, they are readily obtainable at low prices. The nickel issues from 1968 to the 1980s can often be obtained for $1 or less!
1504 Hungary Denar with a doubled A in "PATRONA", and doubling on some reverse letters as well.
Hungary and Poland IMO are the easiest (and least expensive) Western Arabic dated coins in AD.
[Edit: I changed my coin and pic]
If I remember correctly, there are some "brass" zinc'ers that are the result of zinc getting into the copper for plating, resulting in a brass plating.
There's one coin I've expected to find, but have never found in circulation. So I've going to release these into the wild over the next 2-3 weeks in hopes that perhaps some kids will come across them:
I believe the Chamber of Commerce pieces (50c-2F) are technically tokens, but as it was basically the coinage of the day, it has become accepted as part of the regular coinage series.
Now that last one's weird. As a cent, sure, but as a nickel?!
Myself, I used to keep the EUR2c as pocket pieces until I lost a bunch because I kept on accidentally spending them as cents.