In a given dataset, a column of numbers can represent a categorical or numerical variable. While one can imagine some simple heuristics for guessing which (e.g., it's categorical if the number of unique values is less than some fixed threshold), I think it would be a fun exercise to train a classifier instead. Features to pass to the classifier might include the number of unique values and the total number of values.
The only difficulty is that I need a labeled data set of such features. It's probably too much to expect that such a meta-dataset already exists, so my question is: are there open data repositories that are sufficiently uniformly structured that I could pull the datasets and assemble the features in an automated way?