Greyhound racing.  There is absolutely no reason why this couldn't keep going behind closed doors.   There would be less than 50 people at the track.  Social distancing isn't remotely a problem.  No human would have to go without 2 metres of each other throughout the entire meeting.   No jockeys racing like on the horses.  
Likewise we could start the Cricket County Championship in April, with the usual 2 men and their dog in the crowd. 
One day field spread out. (Obviously no standing up for the wicketkeeper  

)
Non striking batsman runs on the next strip. Umpires stand back from the wicket, as bowlers bowl from an angle.