Merging information of rows with the same date
As @Emre has pointed out in comments, you need a pandas custom aggregator.

So since you need a string custom  join by /. Create a custom aggregator as 

foo = lambda a: "/".join(a) 


(or if you need spaces around the join)

foo = lambda a: " / ".join(a) 


Then make a pandas groupby as 

data_.groupby(by='date').agg({'muscle_time': 'sum',
                              'stretch_time': 'sum',
                              'cardio_time': 'sum',
                              'muscles': foo}).reset_index()


This should get you the aggregated dataframe.