well, i'd like to suggest 4 pretty common things first (in the order of importance, i guess):
relay, for more fun electronics,
any kind of getting information from the world without direct interaction (so, sensor): either pressure button or laser emitter+receiver,
some kind of signal delay item, maybe even with delay setting,
anything moving, like elevator or more doors
i understand that moving things will cause even more lag, which is not something people want, but making some automatic doors and light would be hella great