Sentence completion was pretty helpful. You can turn other sorts of prompts into sentence completions eg 'I have felt most grateful when...' The advice for these is to answer them rapid fire multiple times. The first few completions clear out your cached answers.

Sometimes I like to envisage conversations I am likely to have over the coming day or two. I think about what I am hoping to get out of a conversation. I think about what the other people involved in the conversation will be hoping to get out of it. I think about questions I will want to ask, and I think about questions which I am likely to be asked. Etc., etc.

I write down various notes: topics of conversation, questions I want to ask, stories I want to tell, answers I may give, etc.

I enjoy doing conversations with my past or future self.

What would past-me think if they could see what I'm up to right now -- what would surprise, delight, or disappoint them? How do I justify those decisions, and do those justifications stand up to their scrutiny?

What will future-me wish that I'd considered about decisions that I'm currently making? Is there anything about the present that they would ask me to do differently?

Of course they're both just mental models or shoulder advisors, but they're still mostly alright to talk to -- and if they aren't, if I'm being or becoming someone whose company I wouldn't want to keep, that's a useful signal in itself.

One prompt that often helps put things in perspective, though it's more of a thought experiment than a writing exercise for me personally, is to show my mental model of my distant ancestors around in my everyday life. Trying to figure out how I would explain mundane things to them often shows me aspects of those things which I had not considered before.

The book 'The New Diary' by Tristine Rainer has a variety of tools and methods for precisely this expansion of journaling (which go much beyond prompts, so if you're looking narrowly for just prompts then it may be the wrong level of abstraction), and claims that these tools and methods in fact constitute a basis set for the activity.

(FWIW, the book was updated multiple years after its publication, and the author claims that she hasn't found anything not covered by the intervening years not covered by the basis.)

I use a variety of things. Quotes (with a thought of my own usually) collected, pages from daily-reader books, Aurelius's Meditations… I put [one such sequence together as a PDF](https://constantine.name/2020/12/31/practicing-reflection/).