First of all, Lighthouses are an attraction. Most of them are not any longer in use. They can be visited from outside only, occasionally from inside. Some of the Lighthouses have a dramatic location, some a touching history and others are simply beautiful.
To beat the crowds visit between arrivals of the ferries. During winter only open on Wednesdays.
Peggy's Point is world famous and very popular. Tour buses and rental cars arrive throughout the day especially in the summer. However, there are two easy ways to beat the crowds even in the peak months:
Be there in the morning as early as possible. At least before 9 AM. Experiencing the sunrise is amazing anyway.
Don't leave before or right after sunset. Just stay! Most visitor leave when the sun goes down, but the place gets even prettier. Check sunrise und sunset times here: Halifax Canada | Sunrise - Sunset