Don't forget the bearings!
Each wheel requires 2 bearings, so 16 bearings are in a full set. The bearings site inside the hub of the wheel and are responsible for the free spinning of the wheel. A good set of bearings will last a long time, depending on use and abuse.
New skates, sold with wheels, will include the bearings
A bearing puller is an important piece of gear. You need to pull the bearings for cleaning or to swap wheels. A bearing press is easier to use, but costs more! Replacing bearings takes some practice and elbow grease!
When buying a second set of wheels, it may be worth the cost to buy a second set of bearings!
Like wheels, there are many options, but a very common option is Bones Reds Bearings. If using standard wheels, the 8mm bearings should fit!
Bearings also should be cleaned once or twice per year or if the skate got wet. Bones makes a very good cleaning kit that includes everything you need to get started, except the solvent. You will need to purchase a cleaning solution, like isopropyl alcohol (rubbing alcohol) separately. Some kits include the bearing puller!