You last day will depend on your travel plans, so what you do is up to you and your schedule. One option is to sleep in, grab that last tastes of German cuisine and take one last walk around the city before heading home.
Sources
http://www.germany-tourism.de/ENG/destination_germany/master_tlstadt-id1078.htm
http://www.wartburg-eisenach.de/frame_st.htm
http://www.eisenach.de/index.php4?pre_cat_open=153&id=21&new_changed_lang=1
http://www.frankfurt-tourismus.de