My husband and I are planning a 10 day babymoon in Spain in mid March. We're thinking of splitting the time between Madrid and Barcleona. Any recomendations on other iterneraries or things to definitely do/not do?

Babymoon? Which one of the three definitions are you referring too?
http://en.wikipedia.org/wiki/Babymoon
To answer your question there have been a load of suggestion written before. It does depend on your interests and method of travel.
What ever time you spend in one of the locations yo can always take day tips out to see other touristic locations.

What is the significance of a babymoon in Spain? I think we need to know the relationship of babymoon to Spain before suggesting itineraries/places to visit. Perhaps you think Spain is a more fertile country than most?

Sorry- should have been more clear. I will be 24 weeks pregnant with twins- looking for one last hurrah. Typically we are high energy travellers (did Berlin, Copenhagen, Oslo, Bergen and Stockholm in 11 days and thought it was a prefect pace) but I probably won't be up for quite as much as usual. We've wanted to go to Spain for awhile-- just has some appeal as a cool European country. Have not done a lot of research.

Then my suggestion is to split your time between the two cities. and take day trips out to see some others if you are up to it. There has been a lot of suggestions on this forum about what day trips to make. So maybe first do some research to see what fits your likes.