Well Mr Taz, lots of good advice already
I too have been doing road trips to/in Europe every year since the mid/late 80's and all trips have been great.
Personally I think April is a little early in the year as the weather could be very iffy in the moutains, but with less then rigid plans mean changes are easy should need be.
I'd suggest not trying to squeeze too much into your trip as driving can quickly become a chore if you're continually chasing time.
I love the pyrenees and agree that the boat is worth considering if you don't fancy the slog through France. However should you wish to drive through France, it is a truely beautiful country, just need to get off the Autoroute and explore a little. Millau and the Massif Centrale are worth a look.
The pyrenees can be driven west to east or criss crossed between France and Spain for some epic scenery. Andorra is worth a visit, however it is a nit manic in the capital. The N260 is just in the Spanish side of Andorra and is one of the great driving roads. Well worth a visit.
From here you can be in Provence and the Route Napolean within half a day, leading to the Alps and a whole new set of stunning views and roads, this of course puts you where a route home could include the Nurburgring.
Do some research on Slovenia and Croatia as well, I went to Slovenia by bike a couple of years back and was very impressed. Beautiful scenery, stunning roads and a nice relaxed feel to the place. Not been to Croatia, but people I know who have, highly recommend it.
If you're going south as far as Italy/Slovenia, consider the motor-rail from Dusseldorf. You can get off the train at your furthest point south on day 1 - 2 of your hol and spend all the rest driving back.
Numerous roads trips mean many many memories and experiences and far too many suggestions to put on here but any questions, just ask. If you require more info about paticular regions, again, just ask
Just remembered, Berchesgarten, Dachau ............
Good luck fella