Do you own one of those large exercise balls? It’s a pretty affordable fitness tool–less than $20 in most stores like Marshalls or Target. You can use it all the time and work your core without even knowing it. You can watch TV sitting on the ball instead of the couch. If you get one that is large enough, you can eat dinner sitting on the ball–imagine a photo of the whole family sitting at the table on large, varied-colored balls.
It will engage muscles without you trying and probably make you and your family laugh–another good way to engage the core!