The only program that will allow you that type of flexibility is Southwest, and for four round trips, booked online, you get one free.

Most people use a big carrier like Delta,American, or United, and then use Southwest,America West or other as a backup.

If flexibility for same day travel is your biggest priority, then maybe you should consider Southwest as your primary airline, then use AA or UA. For you to get awards on the same day you will have to get a rules buster, and have to use double the miles.

Hope this helps.

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Another evil plot for world domination through the use of points and miles.