Before the start of mowing season is a great time to do things like changing the engine oil and filter, checking or replacing the other filters, and checking any other fluids and topping off as necessary.
It is also a good time to take a look at any drive belts and pulleys. Look for any pulleys that have worn grooves in the belt surface. Also feel for any loose bearings in the pulleys. On the belts, look for any that appear very shiny (this can be a sign that the belt has been slipping or otherwise overheated,) also look for cracks or tears in the belt surface.
If it wasn't done at the end of the season, clean off any debris on the top of the deck. Pay special attention to under the pulley shields as debris tends to build up under there first. Also check under the deck for debris buildup that will affect performance and blade life.
Check the condition of the blades as well. Most blades can be sharpened multiple times. Blades with excessive damage to the cutting edges or wear to the lift section should be replaced.
Tire air pressure should be checked and adjusted to specification. At that point the deck should be checked for level and adjusted accordingly.