All our holidays are flight free. We have done:
Tour de France in the car (Portsmouth- Caen, a couple of days in Chartres with a day trip to Paris, the Auvergne, the Loire Valley, Nantes, Brittany, Roscoff- Plymouth)
Interrailing (Brussels- Innsbruck- Venice- Rome- Genova- Nice- Paris).
Various other trips to France in the car
This year we're doing Plymouth- Roscoff then driving down through France to the Spanish Pyrenees, then the Picos de Europa via Bilbao, then Santander- Plymouth.
I've also wanted to do a tour of the Alps for a while.
If you Interrail (or just buy single train tickets) you can go to Brussels, Amsterdam or Lille if you want to avoid Paris (when do the Olympics end, anyway? I thought they were mostly in July). Lots of TGVs go from Lille- I know there's a direct one to Marseille, for example.
If I was going to go Interrailing with the kids again I'd go in the summer holidays and do Northern and Eastern Europe, sticking to smaller, less touristy destinations (they specifically wanted to go to Italy last time).
Northern Spain is not supposed to be too hot- in fact, I'm hoping it's not going to be too rainy!! If you want to take the ferry there be sure to book good and early though, because the crossings are very popular.