If you want an iPhone SIM then you can rent an iPhone SIM card with data for Germany from Cellhire
. You get 5GB of data with it and free incoming calls.
If you're more interested in taking a data option then you can rent a Cellhire Mifi hotspot with data
for Germany. This gives you 10GB of data and free overage so there is no prospect of bill shock. The Mifi device will allow you to connect you iPad, iPhone, laptop or other phones and devices to it wirelessly but this is just for data - not for voice calls. Although with that much data attached wirelessly you could use Facetime!