Activity/fitness trackers

Hello. I'm looking to purchase a fitness/activity tracker.

Any recommendations for this?

A bit about myself. I'm new to the fitness sceen and working out in general so I'm not looking for anything too fancy. I've used step counters before when I was trying to find motovation to get moving outside of my desk job. They worked for a while but i guess I am gettint bored now and looking for something else. Work outs are pretty much kept to the treadmill during these winter months before I go to work.

I'm thinking of something that could more accurately monitor my heart rate to get a better idea of how hard I am pushing myself during work outs. I previously was going to get a polar watch and chest strap but didnt because my phone broke and money had to be shelled out for a new one. We're coming up to Christmas so thinking of treating myself. I've heard fitbit is also very good but that heart rate tracking is not so great on them.

Any suggestions are greatly appreciated. I'm also not against purchasing a second hand activity tracker if i can find one on ebay or other such website - so please even recommend older models for this newbie