John,
Sailing on the great lakes is nice because it means you're relatively close to sources of parts and supplies, therefore you won't have to stock up very much at all. For example, you'll need only enough oil and filters for one or two changes, maximum. You'll only need a couple of fuel filters since fuel is pretty clean in this part of the world (a boat cruising to Mexico will carry a dozen or more fuel filters). I'd think primarily about spares for things that will get you back to port after a breakage, such as spare hoses, fuses, lamps, etc. You won't need to carry things that are seldom used, such as spare rigging. Instead carry a length of high strength line to temporarily replace a broken shroud -- plus it has any number of other uses. Things that move tend to wear out and break sooner than static things, so bring along replacement parts for the things you use the most. A rebuild kit for the head and fresh water pumps, and probably an impellor for the engine fresh water pump, would be a good start. No need for a spare salt water pump since those give lots of warning as they begin to fail. If you've sailed your boat for more than a couple of seasons, odds are you've got a good start on the sort of inventory you should have for near-shore cruising.
Tools are a whole other thing and what you bring depends largely on what mechanical skills you have and what makes you feel comfortable. If you're a diesel mechanic, for example, then a decent set of mechanics tools will probably find their way on board. At a minimum you'll need all the tools that the various spare parts require for proper installation. It's convenient to have a battery powered drill and various sizes of bits, including screwdriver bits. I have an invertor, thus some of my tools are of the 110 Volt variety. A hacksaw and spare blades is a must, plus cable or bolt cutters in case of a dismasting. You may want some electrical repair tools on board too, such as a 12 Volt test light, multimeter, etc. Since you won't be in remote stretches of ocean, you don't need to be totally self-reliant, so a minimum tool assortment is probably fine.
I feel better with a lot of tools around, so I tend to have more than most people need, including myself.
If you plan to go open-ocean sailing later on, a whole new level of preparation, plus many more spare parts and tools are required.
Good luck in cruising the Great Lakes!