A stopwatch and cheap HR monitor are solid investments. I also have a free Tabata app on my phone that lets me program two sessions - I only use two, one for jumprope HIIT and one for metcons with a sandbag. I only use the HR monitor ( a 20 year old Polar that can't even store data) occasionally to keep me honest or when making changes to an existing routine. Interesting stuff once you get into pacing and rest intervals etc. Every day I jump rope 5 minutes for a warm up, my jump rope HIIT only lasts 12 minutes total with 4 minutes of actual work. I can do an entire resistance session after 5 easy minutes, after the 4 at all out intensity I'm done for the day. If I had to fit in more work after a few hours I'm sure I could, but feel I get best results just letting that ferment. At some point I''ll get into Maffetone base conditioning using HR but I really don't have time for long cardio sessions currently.