Conversation so far:
How did Dennis die?
he fatally drowned in Marina del Rey.
When did he die?
December 28, 1983.
Was he with the band when he died?
Dennis was given an ultimatum after his last performance in November 1983 to check into rehab for his alcohol problems or be banned from performing live with them.
Did he check into rehab?
Dennis checked into rehab for his chance to get sober, but on December 28, 1983, he fatally drowned.
What did the band do after his death?
From 1980 through 1982, the Beach Boys and The Grass Roots performed Independence Day concerts at the National Mall in Washington, D.C..
Did they perform anywhere else during that time frame?
On July 4, 1985, the group played to an afternoon crowd of one million in Philadelphia.
What was the reception of their performances?
the group played to an afternoon crowd of one million in Philadelphia and the same evening they performed for over 750,000 people on the Mall in Washington.
What songs did they perform?
CANNOTANSWER.
Did they perform anywhere else?
The band returned to D.C.
for Independence Day in 1984.
Did they perform in DC after 1985?
CANNOTANSWER.

Question: Any thing else interesting?
Self-contained rewrite: Any thing else interesting besides Dennis rehab for his alcohol problems, death by drowning, and concerts in Philadelphia and at the National Mall in Washington?