Check your air conditioning system. If it needs service, you'll want to have it taken care of in the spring, before temperatrues soar.
Check the cooling system (hoses and radiator) for leaks.
Check the coolant level. Only use the coolant that is recommended in your owners manual. If you're not sure, call your vehicle manufacturer.
In severe summer temperatures, you may have to change the grade of your engine oil. Check your Owner's Manual for the summer oil viscosity grade recommended for your vehicles engine.
Check wiper blades. Last winter's ice may have caused nicks, which decrease performance.
Check the battery and cables. Again, winter temperatures may have reduced its charging capacity.
If you've been driving with snow tires, replace them with summer or all-season tires that have good, deep treads.