A nut driver/skate tool is probably the handiest thing to carry.  They are used for swapping wheels and adjusting action on traditionally designed plates.  Standard American size is 1/2 inch and the standard international size is  

Alen keys can be used to (1) adjust the pinch on toestops and (2) disassemble Arius Platinum plate.

Sweat on the skate floor can be a slipping hazard and contributes to those dirty patches on skates (when skating rinks are humid (especially when there are fog machines)). You can wipe down intermittently or use it as a headband.

• Band-aids

• Antibiotic ointment (Neomycin/polymyxin B/bacitracin (Neosporin))

• Ibuprofen 

You never know when you or someone around will need a little support.

Even if you never use them, it's good insurance 

• Spare nuts (x8) - If you're crazy like me and change wheels a lot, you're bound to loose one

• Spare bearings (x16)

• Spare spacers

• Spare speed rings - some wheels need a little clearance

Now this really depends on if you skate a lot.  I've needed to replace y laces once before; they broke because I had laced up so many times!  Perhaps salt from sweat accelerated the lace's degradation.  All that to say, a spare set of laces might be a good idea to pack when your laces have some age to them.