You already know the answer. You've got just over two weeks left - do Mother's Day right this year.
The Gifts That Keeps on Giving / MakrBox will send your mom a box of "artisinal" treats every month ranging from soaps to spices and back again. Is she a little less Portlandia? Go for the holy grail of beauty product subscription services instead: Birchbox.
Record Frames / Once upon a time, your mom was probably super into music (if she isn't still). It's easy to run to Target or Urban Outfitters and pick some of these up, but go the extra mile and take a basement scavenger hunt for her old favorites. Pop them in yourself so they're ready to hang.
Gospel Brunch / Religious persuasion (or lackthereof) doesn't matter here. This is not church - there may be a choir, but there's also an all-you-can-eat soul food buffet and alcoholic breakfast beverages a-plenty. In short - it's awesome.
A Heavy Dose of Nostalgia / Etsy is amazing for making your mom smile. Engrave a quote from your favorite children's book growing up or some weird experience you two have shared. This one should probably come with a tissue warning, though. Be prepared.
When In Doubt / Make your own brunch, go the the park, see a movie, take her to see Beyoncé or Fleetwood Mac. Do whatever your mom likes but plan it yourself. At the end of the day, she probably just wants to spend time with you. Promise.