i'm planning a trip to central america sometime in april. i want to buy a round trip ticket, but i don't want to be bound to a specific return date. i know i will be gone at least a month, and i really want to leave myself open to staying longer if circumstances allow.

so, are there any search engines i can use where i can leave the return date open ended? or international airlines that allow you to change travel dates after you've purchased your ticket?

any info at all would be greatly appreciated. thanks.