Mrs. Deubster & I have been taking our primary vacations in mid to late September for years. Pesky children are in school, hotels/beaches/attractions are less crowded, the weather is still fine in most areas of the northern hemisphere, and bargain fares and hotel rates are often available.

We love Hawaii in late September - the only drawback is it's too early for either whale watching or spectacular Oahu north shore waves. OTOH, it's the least rainy time of year there.

We've also done Maine in late September for the leaf-peeping, catching the tail end of the lobster season before all the shacks close for the year.

Agree also with the Barcelona/Stiges suggestion.