Can only give you some info as far as Vienna (done this a couple of times), but here goes: you'll probably route via the DVR VOR, then X-Channel towards Calais. If you stay a couple of miles offshore, you can avoid having to talk to Lille App (saves you a very rapid change of frequencies and controllers), then on through Belgium following the 'corridor' between Brussels and Antwerp airspace. Alternative here would of course be to cross northern France, but I've never done that so will stick to the route I know.
After Belgium, you typically very briefly enter Dutch airspace (talking to Maastricht, yep that's right, them of Eurocontrol fame!), then into Germany. Langen Info provide a fab FIS (or RIS, as you'd call it). As fuel stop (both a/c and pilot) I can highly recommend a place called Siegerland (EDGS). Great restaurant and very friendly (and fabulously equipped) tower! While EDGS is not strictly a customs field, they will accept you coming from the UK if you call them just before t/o and give them your ETA. They then call the local police who decide wether to check out some dodgy Brits invading the continent

Best of all, this same police has made the fuellers 'auxillary sheriffs', so the guy who sells you fuel also checks your passport! Great and very relaxing welcome to Schengenland. Add to that the Argentinian steaks in the restaurant....

one word, though: this info is a couple of years old, so check if above arrangements are still in place.
From EDGS you'd be routing through Germany - it's quite amazing how much forest there still is. From the ground one always gets the impression it's all industry and Autobahn. Navaid would be the RDG VOR (beware local glider area), then overhead Passau into Austria. Again, Wien Info provide an excellent service. Basically you've got two options here: either carry on straight following the Danube (nice flight over rolling hills and vineyards further east) or you could swing south and skirt the Alps. I'd say it depends on the day. In Austria alll air
ports will charge you quite a steep landing fee (IIRC, around EUR 45), while air
fields charge a few quid. While the airports have very good service in English, be prepared (at the small fields) for German radio and some, err, unusual joining/landing procedures. Make sure you have your charts/plates/diagrams.
You can land and park a PA28 at VIE (Schwechat), if you fancy flying into a biggish airport.
Don't know the procedures/routes from here, but as Hungary is non-Schengen check which fields you can actually fly into or if you have to enter via a customs field first. Also, recently saw a poster (alas not online, was in the lobby of an Austrian aeroclub) of foreign pilots being charged some outrageous fees (e.g. EUR 50 for passport control) at some Hungrian airfields. Check in advance (and browse online, there's bound to be something about this on the net).
HTH andd have a great trip