I think it depends on your level of need. If you want the light exercise to improve your health, the best option is still cycling. You will get mental comfort and have more space, changing space every day compared to the gym. That is my personal opinion.
My choice is also the chest strap. It's the most popular, reliable, durable, and affordable. But it could make it difficult for obese people (BMI over 30) and women.
Indispensable items for one of my mountain bike rides are: phone, food, water, packable rain jacket, chain lube, first-aid kit, patch kit, tire pump, and also very important and convenient item is the saddle bags.