I love master studies. Find a drawing, painting or design you admire and sketch it. Figure out why you admire it. Analyze what the artist did. And build your sketching skills. Currently, I try to start each practice session with a warm-up master study.
This group of master sketches focuses on robots. Beyond the general theme of “Hey, cool robots!” there isn’t a lot that links them together. I just liked what I saw and had a lot of fun exploring them. You can find links to the original images (along with about 1,000 other images) in this Pinterest board.