There is/are 是一个主语+表语的倒装句型,即把表语放到句首以强调“存在”的句型.其真正的主语是 is/are 后面的成分.
表语提前：there
谓语：are
主语：two avenues to get income for the residents in Stratford-on-Avon,which involves playgoers who are Stratford's most attractive clientele and sightseers who come to look at the sights.其中：
two avenues：主句主语先行词（即被修饰的名词）
to get income for the residents in Stratford-on-Avon,which involves playgoers who are Stratford's most attractive clientele and sightseers who come to look at the sights：不定式结构做后置定语修饰主句主语先行词.其中：
which involves playgoers who are Stratford's most attractive clientele and sightseers who come to look at the sights：非限定性定语从句修饰主句的主语.其中含有两个定语从句：
who are Stratford's most attractive clientele 修饰 playgoers
who come to look at the sights 修饰 sightseers