I haven't seen it mentioned here yet, but I'm a big fan of rowing (Concept 2). I had a rower about 15 years ago, then sold it when I deployed to Iraq for a year. After that, I spent another year in Afghanistan. Then I started my career in tech after I was injured on deployment and left the Marine Corps. Then we had kids—all excuses for not exercising consistently.

I needed something convenient to squeeze in a 15-30 minute workout I'd enjoy once the kids go down, so I got another rower and started rowing again a couple of months ago. I've been pretty consistent (I've worked out 52 days in a row now, with exceptions for special circumstances like deaths in the family). It's a great full-body workout, and I listen to podcasts/audiobooks or watch football if there's a game on. I can't recommend it enough.

Some people have recommended BJJ, which I've enjoyed in the past, but with kids and my wife and I working 60+ hours a week (she owns a business), convenience + enjoyment are the only way to make it something I'll stick to.