To take a rain check is an informal expression which means you can not accept an invitation but you intend to do so at a later date.
For example, last week I was invited out for dinner but because I was working I asked if I could take a rain check.
A workaholic is a person who works compulsively. While the term generally implies that the person enjoys their work, it can also alternately imply that they simply feel compelled to do it
To have a heart of gold is a common idiom which means: to be kind or generous or friendly
EG:My mother has a heart of gold: she is always willing to help a stranger and donate to charity . Furthermore, she has supported me with every good, and every terrible decision I have made in my life. So, do you have a heart of gold or a heart of stone?