व्यायाम करणे

'Vyayam' is a masculine noun. This is why we say 'Vyayam kela' (past tense) and not 'Vyayam keli'.

Yes, but you must specify 'Mansik vyayam' (mental exercise). Otherwise, people will assume you mean physical activity.

'Vyayam' is a general term for exercise. 'Kasrat' usually implies more intense, acrobatic, or difficult physical training.

You say 'Me vyayam karayla janar aahe.'

Yes, in urban areas, 'Workout karne' is very common, but 'Vyayam karne' is the proper Marathi term.

You say 'Mala vyayam karayla avadte.'

No, for football you should use 'Khelne' (to play). You can say football is a 'type' of exercise, but you don't 'do' football vyayam.

It is the Marathi word for a gym or gymnasium.

Ask: 'Tu aaj vyayam kelas ka?' (to a friend) or 'Tumhi aaj vyayam kela ka?' (formal).

Yes, it is the standard formal and neutral term for exercise in all Marathi literature and media.

Generally, yes. Yoga is often referred to as 'Yog-vyayam' or simply 'Yoga'.

There isn't a single word, but 'Vyayam na karne' (not doing exercise) or 'Alas karne' (being lazy) are common opposites.