There are plenty of alternatives for physical activity that don't include spending a lot of money on memberships or stuff like that. Just by doing a some bodyweight exercises like pushups or pullups or burpees it's a great way to do eighter cardio or trenght training. Also, you can run as much as you want without anyone asking for your money. But, on the other hand, signing for a gym membership will give you the confirmation that you will stay on track, because you don't want to waste your money. Also, gym is a great enviroment where you can make new friends and it's a great way to relax.