Collect food wrappers, bottles, boxes, etc. with metric mass, volume, length measurements on them such as shampoo bottles, potato chip bags, etc.
Divide the students into teams of 3-5 depending on your class size.
I have a 100 mL graduated cylinder, a meterstick, and 2 paperclips(~ 1 gram) on my desk for reference.
Hold up a wrapper such as a bottle and have the students in their group estimate how many mL it would hold. Go around the groups and let them bid like on the first part of The Price is Right and give points to the team that comes the closest. I usually have 4 teams and give 3 points for first and 1 point for second. I go around the group and make a different spokesman for each item. With the next item, start the estimates with the first place team from last time. I also use items in the room--how many cm high is my desk, how many cm wide is the doorway, what is the room temperature in Celcius. Their estimates get a lot more accurate over the course of the class period.