I've seen various translations for the phrase es sich gut gehen lassen, including:
to take care of oneself
to have a good time
Can someone provide a definition (or definitions?) of this phrase, and, hopefully, usage notes?
This is the common usage:
Im Urlaub will ich es mir richtig gut gehen lassen.
In Germany, there's a special kind of additional vacation you can go on every few years on a prescription of your doctor. It's called Kur and you usually do this in a spa town. These even have it in their name, they are called Bad …. That's why es sich gut gehen lassen can also have the connotation of taking care of oneself.
So to have a good time isn't exactly what this phrase means, as it usually excludes anything stressful, as going to amusement parks or taking your children with you.
Dressing up nicely and visiting a Casino (Spa towns often have those) may in contrary fit into that phrase.
As Janka pointed out the common phrase usage, I want to point out that both of your translations can fit and not only taking care of yourself.
In general it means "what I think is a good time for me and/or my body I will do". Nothing you would do on a daily base, nothing you could effort on a daily base.
It still could be that someone enjoys this "gut gehen lassen" for a whole year, it still is a luxury living way compared to the rest of the life thus seldom.
And could mean anything else then luxury in money terms because for some people simply taking / having time to do something they want to do (in their dreams) would be "gut gehen lassen".
Let oneself have a good time. Compare to "sich was gönnen" or swedish "att unna sig något".
To allow yourself (to) something nice or tasty or healthy (which might be expensive or otherwise questionable).