OFF! is a popular brand name for insect repellent; you can get it from grocery stores.  I reserved a car in advance...not sure whether they will charge the same price if you rent in local...but in general, Hawaii car rental is very inexpensive .  Last time when I went there, it was also July.  The airfare cost me $500 something, because I bought it only a few days before the trip.  You might find some deals if book early.