It depends on what your relatives want to see. You can actually visit Intramuros for a vintage feeling and eat lunch there, or if they have kids you can go to Oceanarium or Mind Museum at The Fort. You can also bring them to KidZania. :) For adults you can give them a quick tour of City of Dreams and maybe play a game in the casino :)
Tagaytay is a pretty nice place, and although you may encounter some bad traffic going there, especially on weekends and holidays, it's worth the visit.