Any suggestions for turning a birdbath into a planter?

Plant sedum in it,which basically is maitenence free

Make sure whatever you plant has shallow roots. Birdbaths are almost always quite shallow. Mint may be another good option, since it needs to be contained anyway to keep it from totally taking over. It would look gorgeous spilling over the edges.

Hens and Chickens do well in a birdbath or any succulents.

Have you thought about decorative stone, etc in the birdbath & potted plant or plants & flowers on top. Something that cascades down over the birdbath like ivy or vinca vines would be nice.

Here you go...

My birdbath developed a crack three years ago, we tried repairing it with a products from the hardware store to no avail....so I bought succulent soil and some sand, put a layer of sand down first, then the succulent soil and planted a variety os succulents, here two years later it is truly beautiful! There are so many succulents to chose from , try to get ones that will not grow tall, try different shades of green, plus some that bloom, so many shapes, colors and varieties to chose from that you may be like me and get addicted to succulents. We had a eight year drought and were on limited water use, the succulents did great. All the nurseries around our area have many plants to choose from.

I by a potted hanging basket cut off the plastic hangers and set it into the bird bath...something cascading looks better

I filled mine with soil and planted succulents in it

If the birdbath is deep enough go ahead. If it is very shallow like most birdbaths, you are taking a chance planting anything.