My understanding is that in one of the directions (I can't recall which), a passport is compulsory when travelling by air. Therefore, if you fly you must take your passport even if you have NEXUS.
By land, any WHTI-certified identification is acceptable to cross the border including a passport card (US has one), compliant driver's license (some provinces and states have them), or NEXUS/SENTRI/FAST cards. No passport is necessary if you have a WHTI-compliant identity card with you, and you're crossing by land.
I'm not sure how it works for sea or lake crossings as I've never done one.