You can try the zoo (check for times since they do have schedules for Women only days etc...)I am not sure if u have kids, but there is also a play park. Its a nice walk and some people sit and relax on the grass.

There is a gorgeous park behind the Villagio and Hyatt Plaza, at the Aspire zone near the stadium. Great for a walk and picnic.

There is an art Gallery which i need to see also.
There is an Islamic Museum at the corniche.
There are the Boat rides at the Corniche. Short but nice.

You have also the Souk which u can take a walk and have a coffee, shop or eat diner.
You can get a Marhaba guide book or a Qatar Explorer book which gives u info for all you can do.
good luck.