[jemray]

The actual question is to show by half-reactions why it's not a good idea to store a solution of zinc nitrate in an aluminium container. I'm to show this using half-reactions. I guessed that the full reaction would be:
     Zn(NO3)2 (aq) + Al(s)  -->  Al(NO3)3 (aq) + Zn(s)

However, I'm not sure how to divide this into half-reactions, and how doing so would show which metal is more reactive.

[DevaDevil]

there is electron-transfer. The zinc gets reduced and the aluminum oxidized.

So the half reactions would contain 1 of these metals each and there will be electrons. In one half-reaction they get "produced", in the other they are "used"

Try to set up 2 reactions, one starting with Zinc nitrate and ending in zinc metal w/ nitrate ions, the other starting with aluminum metal and nitrate ions, and ending in aluminum nitrate.
Do not forget to balance charges!