You can get along with C/A alone, but in principle only below 5,000ft, some places even lower. With EASA rules every airspace with transponder mandatory is equal to Mode-S mandatory, so beware of the many TMA, TMZs. Also, many CTR will not let you enter by same reason. Flying to uncontrolled airfields free in airspace should work. You have to make sure in planning that you do not rely on any controlled airport, as CTR clearance without S may be denied.