Mother (or father) and baby/toddler exercise sessions.
'Older people' group sessions.
Stay at home moms are your biggest untapped customer group. They are the ones who have the time to use your facility during the day. Consider programs for them, such as yoga, pre-childbirth and post-childbirth exercise programs, dance, discounted exercise memberships, spin and aerobics classes, and so forth. Be prepared to provide some child care (look into licensing requirements).
Offer a 50% discount to customers who use it during the day, you will be flooded with bored housewives who want to lose weight at your gym on the cheap.
promotion / discount
diversify, think of all the things you can do with the room
Basement gym with open floor space, busy in the evenings, not so much during the day time, any ideas of what to do with this space in terms of it being used and being profitable?
