Gallipoli and Troy are the two reasons most visitors come to Canakkale, and they require genuinely different approaches to visit well.
The Gallipoli battlefields, site of the WWI campaign, are spread across a large peninsula — a guided tour is strongly worth it here, since the historical context (troop movements, the sequence of events) is difficult to piece together from monuments alone.
Troy, on the opposite side of the strait, is a UNESCO World Heritage archaeological site — genuinely fascinating for history-minded visitors, though the ruins themselves are less visually dramatic than Ephesus, so calibrate expectations accordingly.
Both sites are commonly visited as a single long day trip from Canakkale city, or as a stop between Istanbul and the Aegean coast for travelers driving the western route.