Looking for advice on the best way to get around Germany and neighbouring countries within a 2 week timeframe and arriving in Frankfurt. Would like to do Prague, Brussels, Berlin, and maybe Auschwitz?
What would be a realistic route?
That's 4 countries in 14 days, or as many people suggest, just 12 days as one loses the first and last day to arrival and departure at the airport, etc. So, 3 days per country. Not very much. I would drop Brussels unless you really want to see it. Berlin is your biggest city and one can easily spend 4-5 days there. Sachsenhausen is also nearby, so you could switch that out for Auschwitz since it doesn't look like you want to see anything else in Poland.
I enjoy riding the trains and think it is the most cost effective. Driving cars from country to country costs a lot and gas is expensive in Germany.
Driving would depend on how many people you are travelling with, there are usually 3 - 4 people in our group and splitting the cost of the car and gas works out fine.
I'll second the suggestion to drop Brussels. It's pretty far from the other places you want to see.
I'd also suggest taking the train. You don't want a car in Berlin and Prague, and many car rental companies won't let you take their cars into the Czech Republic, or charge you more if they do allow it. Same for Poland, if you're going to Auschwitz. It's easy to get a train from Berlin to Prague, and you can stop in Dresden on the way.