My TT has to be used as a mini MPV on lots of occasions.

The Audi baby seat uses the ISOFIX mountings which are very easy to use - Installation

Its designed for 6 months and up to 4 year old children.
The seat is in securely without using the cars seatbelt fittings.
A large soft pad holds your baby in and also makes a nice place for baby to sleep if they are tired.
We keep a soft blanket handy to fold and put on the pad to lift her head higher when she sleeps.

The seat has a seat belt holding the pad in place and a buckle either side making it convenient to clip/unclip the pad.
The only drawback is the general design thickness, if the backrest were thinner and the seat base not so far forward
then our daughter would be able to drop her legs properly and my wife wouldn't have to have her seat all the way forward.

I am considering a base modification but it is a safety item and my wife is short so we'll live with it!

 This is the original Audi ISOFIX

Here are some early age baby seat options from Audi  Birthseat1, Birthseat2

This is the lie flat which I have no experience of but this looks a good option for the back of a TT.

Here is the newer Audi ISOFIX

Here is the Recaro Seat (9+ months)

A pushchair will fit.. if yer lucky! :o)