If your buddies are not so great at estimating time (while on the sticks), an egg timer will keep them honest.  Lack of invites will have the same effect. 
  
A spare oar lock is a must and a breakdown oar is a good idea too.  
Head lamp with spare batteries.  
A garbage can so plastic bags, cans, old leaders etc. don't blow all over the river - the small narrow recycling bins work well (doubles as a bailer). 
Garbage bags to pack out others people's plastic bags, cans, old leaders etc. you're going to find. 
Spare trailer light, bearing, electrical tape/wire nuts, Crescent wrench for bush fixes. 
Toque/gloves. 
Common sense.