Takes a bit of setup on the DM's part, but have the party get ambushed before the end of the day when the wizard's spellbook is getting towards depleted.
The wizard (should) still have lower level spells to help out, but without being able to sling high level stuff around, the non-spellcasters get a chance to show off.
If the party is taking things so slow that they only have one or two encounters a 'day', there should be in-game disadvantages for that too....
It's the min/max thing really, the wizard wants his spellbook as full as possible all the time so the DM has to mix things up enough that he can't always do that.
D.