“活儿”的意思跟“工作”差不多，多指体力劳动。我自己觉得“活儿”这个词是比较口语化的（但我不是母语者）。“忙活儿”就是“忙工作”的意思，也就是说他们在温室里工作，比较忙。 活儿 is roughly the same as 工作, but usually refers to manual labor and has a more colloquial feeling about it. So, they are basically busy working in the greenhouse at the moment...

Someone is busy working ,usually the work is very tired ..

忙活儿=做事；干活儿；做工作……（北方口语） 他们在家跟你舅妈忙（做）温室里的活儿（工作、事儿）呢。

儿是北方地区的儿话音，不带意思，只是强调忙活，忙活是指在温室里面干活的意思

simple mean is "work".