Nobody has any ideas for me? Yikes, I usually get a response! Now I am scared.

I would do some sort of a raised garden all along the side of the ties down the slope. You could build boxes and fill with soil and raise them to at least the top level of your yard or have different heights of boxes. Flowers and/or veggies would grow well because it looks like there is plenty of sun.

There are good systems of stacking block at your home store that will replace the railroad ties - you set them on a level bed of gravel. You can build stairs through them at the end of your extended patio area but seems the best area for play is the sideyard that appears level. You might discover where the water runs below now (look for signs / watch when it rains) and tell the water where you want it to go. You can make a dry creekbed with rounded river rock and place some native riparian tree plantings along it . . it could curve from this side of the stairs to the back corner of the property and you might plant bunch grasses (needlegrass, see what is native there so you don't get overrun with invasives) all around it.

All along the top of the new retaining "wall" you could plant shrubs so people won't walk off the edge, or transition to something higher like groundcover people don't usually walk through - something that is trailing is charming - like rosemary that will spill over the wall or you could build it higher and make it a sit wall - if you take a wall like this over 3' you need a permit and engineering. Sit wall height is 18", so if it retains more than 1' a sit wall is not a good solution.

Don't get the red tinted options / it will clash with your homes red brick. Stick with the earth toned choices. If you have a block supplier in your town, they will have several options, some of them better looking than the others / not curved / etc. Don't default to the home store options until you look at all the systems. There are some nice ashlar pattern options that are much more attractive than the default curved stack.

If you want to make a larger / more level yard right behind your house, you will need to move dirt from another area in the yard or truck it in and it may change the drainage / fence for your neighbors .. . you can't put soil up against the fence and expect it to last . . .so you can shift the line now but not without retaining somewhere . . you could fill to within just a few feet of the fence and then plant the entire fence line with columnar greenery so the lower area is smaller and fille dwith a hedge, but fill dirt is usually not cheap or free.

If you consult a landscaper, they may be able to work with what dirt you have, move it around so the drainage and fencing remains as is, but shift the high areas and low areas to better serve your needs. This is known as cut and fill - if you don't like the straight area, you have a lot of side yard that is flat that you can cut into and fill behind to a point and then transition to a lower area at the side - you could do this with a curve so it is charming . even let the retaining wall be a sit wall for a new lower area in the big side yard / add a fire pit or a kids climbing structure / but you will be replanting or resurfacing every square inch that you dig up and move.