Grunfeld vs Nimzo

Keep in mind that besides the Nimzo, you will need to also look at least one of Bogo-Indian, Queen's Indian or Benoni Defense if white delays Nc3. May have to look at the Catalan as well.

I’m after an unbalanced response to d4, that offers sharp play but some semblance of plans and constant ideas. There are some lines in the Grunfeld with too much memorisation needed. Shame, because it’s a great response to d4

I combine the Nimzo with the Semi-Slav (1 d4 Nf6 2 c4 e6 3 Nf3 d5, not mentioned in #2 but a lot of players play this way to get a QGD with the f3-Nge2 Exchange Variation avoided), but unfortunately that doesn't seem ideal to you because even though there's definitely semblance of plans and constant ideas in the Semi-Slav (dxc4 followed by b5 f.e.), there are definitely some lines that demand memorization. If you're after sharp play you should always memorize some lines, though. When it becomes too much is entirely up to you.

The Nimzo is an extremely healthy choice because in every variation there's multiple ways for Black to play, so you can customize your Nimzo repertoire entirely to your own preferences. 

He could also opt for the Queen's Gambit Declined after 3.Nf3 (e.g. 3...d5)

If you want something offbeat and with a lot less theory than the Nimzo, you could check out the English Defense: 1. d4 e6 2. c4 b6. Of course, you have to be willing to play the French if White goes 2. e4.