I have 2 suggestions.
#1 (the easiest) Build an application that can be used to track donations and donors for a non profit (this was actually my first application I built). You will need a database, a table for donors and another table for donor activity. Keep in mind that donors may donate more than once, so the activity table has to be set up as a One To Many relationship with the donor table. Google if you don’t know what that means.
#1 (much harder) Build an application that can track students, grade, teachers and parents for an elementary school. There is definitely a need for this software so if you do it well, you may be able to market it.