Two or more veg in one space!

I feel another experiment coming on!

There are a couple of techniques to make the most of a small growing space:-
Intercropping - growing a row of fast growing veg between a row of slower veg - so you can harvest the fast ones whilst the  slow ones are still growing
Interplanting - much the same except the fast veg are planted between the slow ones in the row.

If you're crafty, you can do both and get 3 veg from the same space. Something like a row of parsnips (slow) with radish , marking the row and lettuce between the rows.

I remembered all this when I was sowing some carrots in a planter today - how can I grow more in this pot? Maybe there's more to consider than whether its a fast or a slow grower - its the way that they grow and the space they occupy. What if you grew a root crop, a shoot and a leafy crop. Say carrots, spring onions and lettuce together. The carrots need the root space but have airy leaves, spring onions are just straight up and down and lettuces can be as small or leafy as you choose. 

So that's my plan, 3 veg in one pot - 2 quick ones and one slower one. 

Combination suggestions welcome.