Hi Claudia! Below is the floor plan for your space, along with some inspiration pictures as to how I thought your space could turn out. Let me know what you think, and don't hesitate to let me know if you have any questions.
Reception/Retail
This is the front of the building, with an open area plan for the reception, waiting area, and retail displays. I opted to not have a wall separating the entry way to the main work-out space for a few reasons, but most of all for lighting reasons; allowing natural light to flow all the way back will keep a nice open feel and allow the outdoors to 'come inside.' Permanently separating the two spaces will also make both areas feel smaller than they already are.
The reception desk would be two-tiered; A bar-top height section facing the front door used for signing paperwork, paying, etc. The lower section branching off would be normal desk height for regular business operations in the style of the desk above.
The higher portion of the reception desk facing would most likely have to be built. After wrapping the desk itself in a reclaimed pallet wood, I thought for the front of it could be covered in a crawling vine to bring some greenery into the space. You could use real plants, but it may be easier and more economical to use faux plants for this. That way you can get a full and lush look without having to wait for it to grow or worry about it having to receive direct natural light.
Instead of two loveseats, I like the combination of this sofa and matching loveseat. The exposed wood frame and button tufting adds more mid-century charm to the industrial look of the previous loveseats. Plus I really like the lighter colors and their height off the ground. They will help make the area feel lighter and more spacious.
For retail display reclaimed wood and wrought iron will go a long way in giving you the rustic industrial look you're going for. A taller piece for larger items, like shirts, yoga mats, etc. A smaller shelving unit can be used for smaller towels, yoga blocks, and any other knick-knacks you decide to sell.
Main Gym Floor
This is the main workout space, right behind the entry and reception. The equipment shelves would be made out of the same iron piping and reclaimed wood that the retail displays are, and feature cubby spaces under or on either side for storage of personal effects. The large south wall would be your main mirrored wall, with the barre mounted to the floor instead of the wall so you don't have to drill through the glass.
Since there are no full walls to separate the spaces, I thought simple pull curtains, large sliding panels, partition walls, or a combination of any or all of these items, could still provide an open feel for the rooms, but still supply some privacy when needed during certain classes.
There are two walls where I thought back-lighting the space behind the mirrors would provide some really neat lighting opportunities. Putting a large mirror in the space between the equipment racks would provide extra viewing, but would also make the room appear larger by giving the space a 'hall of mirrors' effect. The bathroom wall (the one that's facing the front of the building) could also use one of these back-lit mirrors. During evening classes you would have nice, warm, low level lighting, but it would also help reflect natural light into the space during day-time classes.
To add lighting to the space, I think it will be beneficial to leave most, if not all, of the existing fluorescent lighting mounted to the ceiling. It will provide the brightest and cleanest light for most activities. What I would do is add lighting as pictured above at random heights and in various places throughout the main workout space. They will add a lower-level, warmer light for evening yoga classes that don't need to be bright at all. There are LED bulbs made to look just like old Edison style bulbs that will save energy in the long run.
Bathroom/Kid's Space
The back half of the building will take the most construction (because of the bathrooms) and demolition (because of what WAS the bathroom). With the addition of the water heater to the space with the HVAC, I didn't think you'd really have enough room for an office, but removing the shower like you are thinking about doing will create enough space for a small mini kitchen. I also included a storage closet that you'll need for various office supplies, cleaning supplies, etc. A small mini fridge and retail shelving could be added for sales of water and snacks.
In place of the old bathroom would be a U-shaped bench for kids to sit, and built in cubbies below for bags, toys, books, etc. The equipment shelf between the seating and HVAC closet would be in the same style as the shelves up front, just a bit smaller in scale for the kids to be able to access easily. Moving the seating to the outside keeps the middle of the room open for actual working out. If the bench seating was in the middle it could potentially create issues with different classes waiting for their class to start being a distraction to the current class working out.
Another mirrored wall and barres could be used, or several separate walls with lighting features behind them similar to the mirrors up front.
For the bathroom (or bathrooms, if you choose to add the shower in the end), keeping it as simple as possible will be the way to go. We could do one wall of reclaimed wood, but I think it would be best to keep a simple and inexpensive subway tile on most walls at least up to chest level. That way the walls and floors can be mopped at the same time to make cleaning easier. More industrial pipe shelving to mimic the looks of the gym, and old warehouse-type lighting to bring it all together. the floors may need to be tiled over, depending on how the current holes and gaps in the existing concrete can be filled and cleaned up.
Instead of running wood all the way from front to back, I thought it would be great to tile the bathroom area. Cutting away some of the wood and mixing it in with the tile will give the building a more industrial, lived in feel; like the wood has been there for 60 years, and as some of it peeled away, revealed even older tile underneath.
Changing the flooring material will help delineate between the spaces easier. You could also incorporate more curtains/sliding panel walls to help close them off as well.
To continue bringing the outside farther inside, I thought including some writing or designs (or even a full green wall) with some faux moss and greenery on the wall opposite of the bathroom doors would be a really great accent. It will certainly help the smaller, darker areas feel more connected to the outdoors.
I also thought about incorporating some playful turf flooring under the U-shaped seating and equipment shelf in the kid's workout room. Keeps the outdoors moving inside, and could be fun to incorporate into workouts.