Lorraine Tremblay has compiled a WWW page with advice and suggestions
about ferret treats at <http://www.storm.ca/~ferret/mctreats.shtml>.

Most ferrets enjoy some fruits and vegetables.  Although they're
not necessary for good nutrition if you're feeding your pets a
high-quality cat food, small amounts of these won't hurt.  Just be
sure you don't fill your ferret up on fruit, since he'll need to eat
his regular food to get the required protein.  Too much of nearly
anything can be harmful, so try to vary your treats.

Some popular suggestions: a slice of banana (mashed, so it's more
digestible), raisins, peanut butter, bits of pear, peppermint (small
licks), freeze-dried liver (sold as cat treats), Pounce cat treats,
puffed rice cakes, green beans, wheat crackers, Ferretone, Petromalt
[6.2]...  Try feeding your ferret pretty much anything, in small
pieces.  You never know what yours will consider a fabulous treat.
I've heard of ferrets going wild for everything from spaghetti to
blueberries.

Although most ferrets love milk and ice cream, they shouldn't be
allowed to have much.  This is especially true for young kits, since
the lactose in cow's milk gives ferrets diarrhea, which can easily
cause them to become dehydrated.  Goat's milk, available in some pet
stores, is okay.  Likewise, I've heard that soy milk is good for them
and generally liked, but I haven't seen any verification.

Too much fiber can also give ferrets diarrhea, so limit raisins,
bananas, prunes, oatmeal, apples, and anything with bran in it.
Sugary treats aren't good for them either, since they can cause
dental problems.  (Despite the rumors, there is no evidence that sugar
causes diabetes or other metabolic problems in mammals.)

Be careful with chocolate.  Most ferrets like it, but the
xanthines/theobromine found in it may be toxic to them in large enough
quantities; nobody's sure.  It's not recommended as a treat.
(However, many people give their ferrets an occasional chocolate chip
with no problems.)  Likewise licorice -- the real thing, not the
plastic, fruity, red stuff that goes by the same name -- is
surprisingly strong.  It's been used for medicinal purposes in the
past; it might not be a good treat.  Both chocolate and licorice are
more likely to be dangerous to ferrets with heart problems [1.1].
Onions, garlic, and other members of that family can cause Heinz body
anemia in dogs and cats; nobody's sure about ferrets, or what the
dangerous dose might be (the tiny bit in some meat baby foods is
probably fine), but caution is advised.