Here is an exciting example published last year - though I'm sure your opponent will say a "snail is an snail":
Evolution: single-gene speciation by left-right reversal.
Ueshima R, Asami T.
Nature. 2003 Oct 16;425(6959):679.
The researchers witnessed a speciation event in a closed population they were studying, a single gene mutation changed the shell pattern of a snail, and the constraints of the new shell shape prevent the snails with the two types of shells from aligning their genitals to mate. But, the old-shelled snails could mate with the old-shelled, and the new-shelled could mate with other new-shelled snails.
Thus snails with the shell-changing mutation are reproductively isolated from the ones without the mutation - even if they are sitting next to each other in the same pond.
Geographical isolation is not required for reproductive isolation - another example is different species of crickets that could mate based on reproductive biology, but never do because their song pattern (mating behavior) doesn't match.
I think the snail example powerful: Humans witnessed it, it is based on a single gene mutation, that mutation prevents mating between those with and without the mutation (reproductive isolation), and the shell pattern is visibly different (morphology difference).
The problem is - to a creationist, a snail is a snail is a snail (is sometimes a slug...)