I say, make it clear that you are being friends with her- so doing something like an amusement park would be fun, but I'd suggest bringing other people who aren't couples along too just so this girl doesn't get the feeling she's on a "double date." It sounds like she likes you but doesn't know what she wants- I know a lot of people keep their high school boyfriends around when they're in college because they dont know what else to do, or because they feel them to be a "security blanket" against getting to know other guys. Just let her know you're there, and be nice to her. She might have some more growing up to do before she feels comfortable in her own skin, and if you're willing to wait for her to do that then by all means, do. She sounds like a really nice girl, and you sound like you're doing everything you can to make sure you're as chilvarious as possible. Keep doing all these things, and make sure you are affirming her wonderfulness every day by assuring her of how smart and beautiful you think she is (but don't go over the top with these things, just say things like "Wow, that was a really insightful observation" and "that shirt really brings out the color of your eyes"). Let her know you're paying attention to her and like doing so, and it will most likely be the catylist to her "growing up" and out of this "keeping my old HS boyfriend around for the hell of it" phase.
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Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi