A fun game is following the reference trail right back to the source works, often decades ago. Pick something popular, say convolutional neural nets. Do a brief literature review and try and read the most highly cited papers as far back as you can go. You'll usually hit some really interesting (and very readable) old papers that cover really groundbreaking things. Using the convnet example, at some point you'll come across the Nobel winning paper on how cat brains interpret visual stimuli (Hubel, 1963).