Some exercises I consider as warm ups are :
Gesture Drawing
Thumbnail
Shape Drawing
Form Drawing
Imagination Drawing
The ones I do the most currently is Thumbnails, the ones I did the most in the past was Gesture/shape/form Drawing.
But I still do them from time to time even if I am focusing mainly on landscapes. The warm ups usually last for an hour or two hours. Bobby chiu talked how he used to do 4 hours warm ups in one of his videos.
But yeah. That's the things I do to get in the drive. I don't always do them, sometimes I just get right into the project I am doing.