I live near you. I have also been a raised box gardener in the Denver area for decades. My current garden boxes were made from a variety of things, mostly recycled materials.
You can start seeds now outside, but there are not many things that will do well alone outside. Results will be hard to predict with our weird weather, and the nature of your raised boxes. I have spinach, parsley, and mustard that are still alive. The production is not much though.
A neat thing about the raised boxes is that you have more control over a microclimate in each box. If your boxes are all the same size, you can make trellises and cold frames that can be moved based on your plan each year. A box with a cold frame will do great things for January and February greens here. Unfortunately, you are a bit late for that. Still, if you could rig one quickly, you would still be able to get a lot of different greens.
I have 8 raised boxes. No 2 are the same. This is different from my prior raised box gardens. So my routine is also changing. I have a basement plant room. I just started things a lot earlier than previously. I have greenhouse cucumbers and stevia that just sprouted. I was hoping I could grow some cukes in my basement. I don't know about the stevia and starting it so early indoors.
I will be planting snow peas and snap peas outside in my boxes next week. If I can get a cold frame assembled, I will start a bunch of greens in the box with the cold frame. I have the parts for the frame, just need to do a little welding.
Are you experienced with raised boxes, or is this your first experience. Please describe how you constructed them. Are they made of wood? What did you do for irrigation (both watering and drainage)? How are the corners made?
My decades of experience with raised boxes in Denver has shown me that Mother Nature will break wood boxes apart at the corners in about 5-10 years and cause them to bow if they are long. Watering and drainage are the other factors that may need some close attention as you build experience.
I took a water bed heater off Craigs list for free and built a 2x10 box around it. I added a layer of plastic and then 4 inches of sand. I dampened the sand and covered it with plastic. That is where I place the seed flats. The warm sand helps with seed germination. Some types of seed need a "paper" planter so the roots are not disturbed with transplanting. With our climate, some plants must be repotted more than once to make it to our last frost date.
What type of plants interest you?
Good luck, Gary.