What to do with a 'free' Data Plan?

Hey everyone,

before the iPad Air release, T-Mobile Germany sold the iPad 4 (refurb) with 64 gigs for 1 € with a 25 € data plan.

I immediately pounced on that, sold the iPad for 640 € and now I have 19 months left of my 'free' data plan. Other than that I only have a prepaid SIM. I was about to get a new contract with a phone, but that seems a bit redundant now. My first Samsung Galaxy S1 just broke, so I'm left with:

1 SIM for a data plan
1 SIM for phonecalls and SMS (Prepaid, usually about 15 € a month)

T-Mobile won't let me combine them or upgrade the data plan to a Call+Data one. I have no devices besides an MBA.

What would you do with that in my situation, as a student / musician?