It is a great day out and I would recomend it. Of course it is all very touristy, and you will be offered loads of momentos, but hey lifes for living.
You do not need to take a bus trip. From Maipu Station you can buy a ticket for the tren de la costa and including boat trip for 25$ar at least that is what it was last time I went, it may well be dearer now, but nowhere near what you will pay for an organised trip.
Saturday is outdoor market day in Tigre- crafts, household goods, and much more.
Saturday is also flea market/antique day at the Barrancas station of the Tren de la Costa, quiet and pretty, worth a stop, with a small beach resort across the street where you can have a beer or rent a bike or a kayak.
If you just go to the Retiro station, you can buy tickets and change trains at Maipu.
Tigre is beautiful. Just 20 miles outside the city, and it feels like another world. Great for breathing some different air and doing something very un-buenos aires like. The puerto do frutos has lots of handmade goods, lots of tat but some decent homewares which are a lot cheaper than you find in the city. Train directly from retiro takes about 50 minutes.
Its a really good quick gettaway if you are getting claustrophobic in the city. Recommended!
The Isla Martin Garcia is well worth a visit. Its a quirky place - overgrown and pleasantly decaying - that rewards slow, meandering exploration. You can do day trips or stay at the hospedaje or camping - the inclusive tickets with accommodation and meals at the hospedaje are very good value.
There is a train de la costa that leaves from Olivos. Tigre is touristy, but mostly Argentina people. If you do go during a Saturday make sure to go early. If you decide to take a boat tour, etc sometimes the last one leave at 14hr or 16 hrs depending on the time. There is a good place to eat near the end of the market area by the river. If you go durning the weekday the area is not as crowed. There is also an amusemunt park and a Casino.
Tigre is a def. worth a visit. The easiest way to get there is from Retiro, you can take the train directly to Tigre, and explore from there! If you want to go to the fair, just ask where the feria is, and eventually you'll find it, great area, good to get outside the main part of the city.
no need to switch trains..take one all the way from retiro. tren de la costa is a rip, very expensive...plus the hassle of changing trains!
once in tigre, either buy a round trip ticket for $15 or you can reserve lunch, always a pleasant day out. el gato blanco is excellent but expensive, la vaca something or other has become run down and the food not so good...i went camping in tigre a month or so ago, in a place called luz y fuerza: $15 round trip boat ride, $12 entrance fee per person , and $12 per night per person...lots of places to put tents.
Don't miss the MAT, Tigre Art Museum, which is fantastic... Housed in a beautiful beaux arts mansion, it has some great Argentine works from around the country... Highly recommended. The walk there from the train station is very nice... along the Lujan River park...