Good follow up to 1.d4 Nf6 2. e3 for black I play the nimzo Indian defence, and so in response to 1.d4 I play Nf6, planning on playing e6 and Bb4. 
However, if they follow up with any other moves, I am at a bit of a loss as to what to play. I tend to just play d5 and develop my pieces, but I was wondering if there was any other good opening I could use in response to that.
The Nimzo-Indian can be closely paired with The Queen's-Indian, or The
  Bogo-Indian, or much less closely paired with the Benoni.


Lets forget about the Benoni for now and focus on the other two options.

In the Bogo and Nimzo you see Black's Bishop going to b4.  The general idea is to fight for the e4 square, but in some cases Black will trade the Bishop off and then put his own pawns on dark squares (d7-d6 and e6-e5).
When you play an early d5 as Black you kill this secondary plan -- which is totally fine but something to keep in mind.

In the Queen's Indian you target e4 with Bb7 and the other Bishop usually goes to e7.  Here you are also fighting for e4 and because you aren't taking out Whites Queen's-knight who usually will attack the e4 square, you need help of a pawn and d7-d5 is often played.


  So, what's with the Benoni?


If White is avoiding c4 and Nc3 then they are playing slowly and it is not critical.
If they play c4 but avoid Nc3, say 1.d4 Nf6 2.c4 e6 3.Nf3 then many players like to play 3...c5!   Since White has put their Knight on f3 a lot of aggressive lines with f2-f4 are avoided.

Nick De Firmian paired the Nimzo with the Benoni.
Karpov paired the Nimzo with the Queen's Indian.
Fischer sometimes played a hybrid of the Queen's Indian and the Nimzo Indian.

The smallest amount of theory to know would pair the Nimzo with the Bogo-Indian.