Coming up on 50 days and don't know how to celebrate sober...
It depends on what you’re celebrating, and the setting. I always make sure I have a sober friend to attend events!
I go to a meeting and chill the rest of the day.
Cake?
I'm overthinking it. Cake and a meeting should work
You caught yourself overthinking it and that the key. I asked the same question. After 30 yrs of use I didn’t know any other way. Still learning
Well as a woman, I treat myself to self care for every milestone. Get a massage, it helps get rid of all the toxins still lingering around. But cake works too
Have a Shirley Temple.
Shirley temples always do it for me..
I think it’s different for everyone. For me personally I had to figure out things that I enjoy and what I find to be fun. A celebration can be whatever you want it to be.
Think about the fact that the alcohol industry has been mind fu©king you all your life, making you believe that you "deserve" a drink after a hard day or that celebrations always call for alcohol. Have a pop and a good time and wake up with no regrets.
Also, I think it depends on what you define "celebrate" to mean. Maybe you need to redefine the word?
I actually celebrate in my zoom meetings because my family could care. They don’t even acknowledge it. 
I like to collect a group of fellow sober people after meetings and go out for fun food! Burgers, hotdogs, ice cream!
Comfort food! Fear no fork
I’m pretty much doing what I always did, but not drinking. I don’t do mocktails. But I will sometimes make a special coffee drink…like a mocha or red eye. I’ve been trying different Ginger Beers(the NA type). I’m trying to just dive into the moment as a new experience and get the most out of it.
Any celebration of any sort can be very triggering for me, so I do “bookending” with a group chat I have with sober people….
We discuss a game plan going into the celebration to hold me accountable (and run through hypotheticals) then I check in with them after I leave. How long will I stay? What will I eat or drink? How will I handle running into certain triggering people (it’s all about managing my stressors)?
Shoot guns
I drink a sparkling mineral water like San Pelligrino. Makes me feel special and I’m enjoying something that is good for me! Plus add in a piece of cake. 
Prime rib dinner and a good dessert