I agree with almost everything you say here, but just a few points.
CorDarei said:
I think your translation for "tell me about your trip" is good, although I might say 話して instead of 教えて. However, I don't think I would ever say that to anyone, even in English; it sounds too blunt, or forceful, or something. If I want to know about someone's trip, I ask "How was your trip?"
Well, but there are some situations where the two wouldn't exactly be interchangable. For example, in English I might say something like "Tell me all about it when you get back (or next time you write, or next time we speak on the phone, etc.)"
In this case, I think something like 「旅行の話聞かせてね」 wouldn't be too blunt or forceful or anything, don't you think?
CorDarei said:
Have fun - 楽しんでってね or 楽しんで行ってください
Hmm, if this is something you're saying to someone about to leave on vacation, don't you think that 「楽しんで
きてね」 or 「楽しんで
きてください」 would be better? My sense is that (and a quick google search for both phrases seems to back up my suspicions) if it's 行く, it sounds more like "have fun (here) before you go." If you're trying to say "have fun (on your trip, and then come back)" then I think you'd want くる instead.
CorDarei said:
Will it be fun? - I would say 楽しみにしてますか? which is really more like "Are you looking forward to it?", but that's what I would say in English anyway.
This would probably be fine if you were genuinely unsure (and wanted to ask) whether or not the person in question was looking forward to the trip or not, but assuming that he/she is giving any signs at all the the trip is going to be fun, I'd probably go with 「楽しみでしょうね」 ("You must be looking forward to it") or (in the right context) 「楽しそうですね」 ("Sounds like it'll be fun.")
Just my two cents.
(
edit-- Oops! Elizabeth's post seems to have come up before mine saying almost the same things about 「(旅行の話)聞かせてね」. Oh well, I'll leave it up anyway.)