Rachelle Gardner offers 11 Completely Non-Writing-Related Ideas for becoming a better writer. You should check out the complete list and some of the additional suggestions in the comments, but these were my favorites:
1. Be creative any way you can. Cook new recipes. Paint a picture. Design a garden. Compose a song. Build something with Legos. Organize the garage.
5. Take trips. Fly, drive or ride your bike. Dig a well in Africa or lie on a beach in San Diego or discover just how big Texas really is when you drive across it… just go.
7. Be a patron of the arts. Theater, dance, museums, concerts, architecture, baseball games. Enjoy beauty, excellence and precision wherever you find it.
8. Spend time everyday in mundane tasks. Do them without any music, radio or TV in the background. Wash the dishes, mow the lawn. Even your commute can be done in silence. See what you find there.