While I admit that I haven't done a day trip to Colonia yet, it seems to get mixed reviews. From what I have been able to gather, it makes for a pleasant relaxing day, but if you have any expectations about it being a great destination with some sort of wow factor beyond the passport stamp, you might be disappointed.

But if it was me and I was only going to be in BsAs for three days, I probably wouldn't waste one of them on the trip (unless I was pretty sure that I wasn't coming back anytime soon).

If you want to keep your options open, just plan to do a same day trip on the high speed ferry on your last full day. And don't buy your tickets until the day before in the unlikely event that you decide that you have seen all BsAs has to offer.