I also loved your little rhyme, and I have waiting all day for a chance to answer this question, but sadly I some things to do for my real life.
Areylia: She actually hates Valentine’s Day, so there probably wouldn’t be a date, and since Adam probably wouldn’t care I’d say nothing happens for these two. I could actually see the both of them working through Valentine’s day while Tina and Felix try their hardest to get the two to go out and do something fun. Her ideal date on a non-Valentine’s day would be something either classy or old-fashioned like ballroom dancing (although that’s a really awkward height difference to dance around.)
Lukas: He would be the one doing the asking, but would plan the date around something Nat would enjoy like going to a museum, and would end it with dinner at a fancy restaurant, and a quiet walk. Lukas doesn’t particularly care what they do as long as they’re together, so it would be more about what Nat wants to do.
Riley: I don’t think Riley would even realize that it’s Valentine’s day unless Tina put up decorations or something, so Felix would have to plan the date which would likely consist of kidnaping Riley, and forcing her to have fun doing something. As for what that something would be, I’m not quite sure since I’m still trying to figure out what Riley likes to do in her free time, except draw, play video games (mostly story oriented RPGs like Baldur’s Gate or Neverwinter Nights), and D&D, none of which are really date activities. Maybe they could go to an arcade? (I also had this image after thinking about video games of Felix playing Just Dance which Riley would never do, but the thought made me smile).
Cat: There is also not likely to be a date here because Cat would never do the asking, and I’m sure we’re all in agree that Mason wouldn’t. As for what Cat’s ideal date would be if it miraculously did happen, I’m not sure she knows. She has like zero romantic experience, so all she’s got to go on are books and movies, and not anything like what she would actually like. Mostly, I think she’d just want it to be the two of them spending so time alone where they’re talking instead of Mason just turning it into sex. She would probably get Mason a present though, and would be sad if he didn’t want and/or like it.
Rhiannon: She would like something more intellectual like going to the opera or a Broadway musical, and I think that’s what she would want to do regardless of if it was Adam or Nate asking, and she would go with whoever asked her first, or if this was after the triangle dissolved whoever she ended up with. (Which I was thinking is probably going to be Adam, but lately I’ve been wondering if they aren’t too similar. Oh, well don’t have to decide yet.) She would also dress up for it, but would be embarrassed if anyone (cough cough Felix) commented on the extra effort put into her appearance.
Eleanor: She would want the classic Valentine’s Day stuff: flowers, chocolate, stuffed animal followed by dinner at a nice restaurant, and an after dinner activity that might be walking in the park or ice-skating. She would be the one to ask for a date, but would fully expect either Adam or Nate (Whoever it was at that point, probably Nate) to actually plan it out, and would enlist the help of Felix and/or Tina to make sure they do it right.