As someone who has done pretty much exclusively 1x1s for years now (and has started almost every reply to an interest check with an apology for my OOC conversation title), I second SleepingSilence's point of not worrying too much about how suitable or sellable the title will be. Granted, if you're writing an interest check for a group rp, then BangoSkank's method seems like a very good idea. Give the dang idea a name and description in the same instant. Pretty useful for people browsing the forum. 

But if that's not a concern and it is purely so you can put up your intro post and get the ball rolling, then I would absolutely go for some specificity when you're considering the title. Using your example, I know that a fishing trip is absolutely taking place so if I use that as a reference point, it won't suddenly become an obsolete point because fishing never happened. Of course, your trouble with specifics still applies, so then I would look for a line or a phrase that has to do with fishing, but comes up in other walks of life. Sayings, for example, like Give a Man a Fish... or Like a Worm on a Hook... or something about the environment like maybe Ripples on the Shore... (that one sounds stupid but you get the idea... it's totally a lakemonster problem, isn't it? zombeavers anyone?)

Granted, this generally works better if you have both an absolute scene that won't change and an absolute genre or theme or plot point somewhere down the line to steer your choice of vagary in a suggestive direction. But there can be some random sayings and just simple phrases that fit a scene and are still so open-ended they'd work for pretty much anything. 

My immediate thought when you said fishing trip was that the title should totally be Hook, Line, & Sinker.

Puns are also great. As is asking your partners for help brainstorming titles.