I have a few friends that work in tourism so I might be able to chip in here based on their suggestions, my own experiences and some hearsay.
BuenosTours have a solid reputation as being some of the best walking tour guides in BA. Good choice and a good way to start the city.
Biking Buenos Aires and
Urban Biking are both reputable biking companies.
I would say Tigre before Colonia. Colonia will chew up an entire day and is "meh" in my opinion. It's good to add a passport stamp and see different money, but it is not that different from Argentina. You need to go further into UR.
Bring your walking shoes. BA is very walk friendly and I find myself walking 5-10 kilometers a day without even thinking about it. San Telmo and Palermo are especially easy to explore. Just stay clear of La Boca and Constitucion.
Keep an eye on
LandingpadBA's blog. They write an article once every 2-3 weeks or so. Useful for keeping you in the know. I've been to a couple of futbol games with them which should be considered as well. The Argentine fans are something else!
I would say depending on the companies cancellation policies book in advance. Better to be safe than sorry. Buenos Tours I imagine will be booked up around that time.
Street food, I'd swing by
Nuestra Parrilla. That same site, Pick up the Fork is a god's send for food info.
Teatro Colon is a given as is the MALBA. Perhaps add on the
PROA and
MACBA. I would taxi in and out for the PROA though. I'm not a fan of the surrounding area in La Boca and you should stick to the touristy section.
I dont know much about tango or luggage storage, but I am sure tons of suggestions are coming at you.