I'm sorry. I just relied to the information on the Fonic website. But in Leipzig there's a shopping mall in the central station ("Promenaden Leipzig Hauptbahnhof" at Willy-Brandt-Platz) which is opened on Sundays and on public holidays from 13:00 through 18:00 and where there is REWE (supermarket), Rossmann (drugstore) and Saturn (electronics). According to Rossmann Filialsuche the Rossmann store is opened Sundays from 9:30 through 20:00 and REWE should be opened from 12:00 through 18:00. I expect them to be opened on Easter Monday at the same time, too.

Thanks, but I am now back in Prague and I'll be going to Germany again on Wednesday noon (to Hamburg, Star Wars concert :) so I presume I can buy the Fonic anywhere at that time.

So, when I buy the SIM card, I first have to find a WiFi somewhere and activate the card over internet at fonic.de and then I have to call the customer support at 80000 (in German) and tell them to activate the flat data tarif, is this correct?

You can also activate your SIM by phone at 0800/4636642 (toll free, working hours: Mon-Sat 8-20 h) if you don't have internet access.
If you activate by phone you could ask the operator to switch to "Internet-Tagesflatrate" already upon activation. Otherwise call customer care after activation at 80000 and request the tariff switch afterwards.

I ended up getting Vodafone CallYa which worked great, except one interesting glitch/feature:

I got a week-long data access and everything in my HTC Hero worked great - Browsing, E-mails, chat, maps, even YouTube. However, several times during the day, when I didn't actively use the phone, all data connections stopped working and I had to visit an internet page (any page!) in my browser - this displayed the Vodafone landing page and from this instant, eveything worked again.