I didn't see this earlier, but the proposed Apollo Venus Flyby... it was such a weirdly fascinating idea among the Apollo Applications Program proposals that were tossed about. It's something I find that is just honestly a rather unique idea that was considered in terms of well, what could be theoretically achieved using existing hardware, but in a broad sense found the original reasons for it eventually superseded by both Mariner 10 and the Pioneer Venus missions (Pioneer Venus Orbiter and Pioneer Venus Multiprobe).