My son's idea of making math fun was doing math while also playing video games, often role playing strategy type games. Multitasking is supposed to be hard for people with dyspraxia, which he was diagnosed with, but I could ask him the questions and he would do the math mentally while playing the game, at least for the easier ones. For word problems, he would pause his game and read the question and answer it faster than I could. He actually seems to do much better in math when he is multitasking because when he is multitasking he can better maintain focus.
He also liked to race me because I usually lost since I had to use pencil and paper or a calculator more often than he did. He has motor dysgraphia so paper and pencil really slow him down, so I am encouraging the mental math.
He can't stand to do a lot of one type of problem so switches to another type of problem when he feels he is losing focus. He sometimes does math in bed on the iPad. He has done over 1,000 7th and 8th grade math problems on IXL since October, most of it using mental math.
He had to work around frequent migraines and other pain that made it harder for him to focus so I did whatever it took to get him to do math. He is working through the Life of Fred Algebra book again now that amitriptyline has cut down the frequency and severity of the migraines. I think it was about a year ago when he started the algebra book but he had difficulty adjusting to his brace and the headaches so he just wanted to read interesting things because it helped distract him from the pain. He did not do much math at all for a year and I was afraid he might fall behind, but he is back on track.
