Yes Tigre is worth visiting....beauty about it is it's easily accessible so not a lot of travelling to do.
The ferry over to Colonia is also worth doing...pick a good day though. As for visiting an estancia. I have no experience of any near Buenos Aires I just know one or two where I live which can't really be done in a day from the capital. The ones here are very old and the real deal....they are still working estancias and pretty rough and ready. I suspect the better known ones near BA will be more geared up for tourists.
Check this thread out about visiting an estancia.

Enjoy your hol. ^