As to something neat to do on her birthday, I once bought tickets to a upbeat musical (Mama Mia) and invited my kids and my husband's friends one year to all go to it together on his birthday. They seemed to love it.
You can memorialize her by buying a "brick" with her name on it to something she might have liked (the renovation of her school, etc).
Create a memorial at your house in a little flower garden with perhaps a large piece of flagstone, surrounded by some evergreens. Place a heavy-enough statue of something that makes you smile (we did an chubby angel child with wings), and I put out a little living perennial plant in bloom on his birthday (and then plant it in the ground the next week so it will come up every year around that time. Callilillies are great I think)
You can plant a tree in your yard or dedicate one in a park you visit where the tree blooms around this time of year (like a Magnolia which blooms that time of year).
If those ashes aren't sealed yet, there are things that the internet (and funeral homes) sells that you can put a small bit of cremains in for yourself (a heart, a jewelry box, a picture frame).
In my closet on a shelf, I have a few fabric covered boxes that look nice together (from Hobby Lobby). You can save the greeting cards to/from her, something of her (sunglasses, ring, bookmark, watch). Perhaps you have a few of those things or could ask for some.
You might also ask your stepdad if he would will you the cremains down the road. The question becomes, how to do you plan what to do with cremains in the event of your death? You can't just think that every generation from here on will want that responsibility. Maybe if you talk to your stepdad about that, you can figure out a permanent place for the cremains down the line.
Other sweet things: Buy a bunch of fresh flowers for your table that will last that week. If she didn't have a favorite, use yours.
Buy a coffee cup for yourself that speaks lovingly or whimsically to you. (Look at the Hallmark store)
Or, have one of those photocups made online with her picture or one of the two of you. The more informal the better.
Make/order one of those photobooks online that have your favorite pictures of her and you and DS and anyone else near and dear to her and you.
Buy a locket for yourself, put her picture in it, and wear it on her birthday week.
Pay for a brief subscription to Ancestry.com long enough to add in whatever info you have for your mom and spend a bit of time looking for her ancestors. you can scan in official documents to formalize the chain (her birth certificate and death certificate, your and your DS birth certificate, your DS). I don't know if you can put in a scan of the obit posted, a picture of the urn, etc. You wouldn't want this info lost and it can be quite fun and interesting for others and your kids and theirs.