I am using Delta FF miles to go to Amsterdam in October. If I can't get a flight into Amsterdam, where are good alternatives. London, Brussels, Paris, Frankfut, Stuttgart. I am not opposed to a train ride.

I would fly to Bruxelles. This is what I did myself back in 1999. The fares from Rome to Amsterdam were way too high, so I flew to Bruxelles. There are trains that connect the 2 cities.

Depending upon the London airport, you can get cheap flights to Amsterdam on Easyjet of Flybmi. Check out this site for which budget airlines fly where:
http://www.whichbudget.com/

Easyjet OR Flybmi. They haven't merged (yet...).

Paris, and take the train from there. I've done it in reverse.

For trains:

Brussels is 1st choice apparently.

2nd would be Frankfurt. You can take an ICE train without change from the FRA airport to Amsterdam in about 3.5 hours.

From CDG, it's with one change, and at least 4 hours travel time.

Not London! Absolutely horrible to transfer on Heathrow. BTW most low-cost flights to AMS depart from Stansted so you''l have to change airport. I'd go for Brussels or Frankfurt. Not Paris, since CDG is a nightmare.

Brussels would be my first choice. Does Delta fly to Dusseldorf? that would be another decent alternative.

I'm fairly certain that you can use Delta Skymiles on Northwest/KLM flights as well.

Northwest flies to Amsterdam.

Don't dismiss London.

Easyjet does Gatwick-AMS. And if you're really on DL, you'll come into Gatwick. The presence of Easyjet in turn pushes BA prices down

In fact, LGW is really the ONLY serious intercontinental gateway with a decent set of low-costs operating out of it. Infinitely better served than Delta's other main European gateway, Frankfurt (which thinks itself far too big and important for these measly little cheapos. German Disease writ large)

Just bear in mind the messiness of transferring to Easyjet (you have to exit with your baggage and chreck in landside), and build that into your timing.