I've stayed at the Hotel San Martin several times when visiting Foz do Iguaçu. I love this place, and that's what keeps me coming back.
The hotel is located only about 200-300 meters from the entrance to the Brazilian park. It's right next to the Bird Park. It's only about 4 kilometers from the airport--very convenient all around.
The hotel itself is situated on a spacious property. The front of the property is very nicely appointed as a welcome area, reception, and the hotel rooms themselves. The whole back end of the property includes a soccer field and nature trails that go back into the jungle. The property actually goes all the way back to the Iguaçu River (considerably downstream from the falls), but while I've strolled the trails back there, I've never gone all the way to the river.
The rooms are very basic--almost rustic in some respects. (They have some newer rooms that have been remodeled more to contemporary standards.) The rooms aren't large at all, but they're kept very clean, and they're very comfortable. The bathroom includes a tub-shower and, in some cases at least, a bidet.
You're not in Foz to stay in your hotel room, though! The hotel is so conveniently situated, you can walk to the Cataratas do Iguaçu national park. A bus stop right across the highway from the hotel entrance is convenient for public transportation to downtown Foz, to Argentina or to Paraguay.
In the hotel lobby you can schedule tours to Itaipú Dam, the Argentine national park, or other sights. Tours offered at the hotel are convenient and are fairly priced for the tourist market, but they are not inexpensive; you may be able to find other more economical alternatives if you're a savvy traveler.
The hotel property itself is fun to roam. Walk the nature trails and enjoy the jungle. The swimming pool is beautiful, clean, and a great way to cool off on a warm Brazilian summer day. The poolside bar and grill will make sure your food and beverage needs are met.
Speaking of food and beverage, the hotel offers complimentary breakfast to all guests. Lunch and supper are available in the restaurant for an additional charge. The food in the hotel restaurant is very good. Evening meals usually include a very well-spread buffet, with a main meal theme (churrasco, etc.). The prices are very reasonable, and the quality of food is excellent. Inquire at the desk, because on some evenings the supper meal affords the option of dining outside on the grassy patio--a delightful experience. Live music is often part of the evening ambience.
In the evenings the lobby often becomes a gathering place for hotel guests, many of them from Argentina. It's a fun way to get to know people and share experiences. There is a game room with a pool table and a ping-pong table. The bar is conveniently located right off the lobby.
I myself have not visited the Hotel San Martin with my family, but I believe the hotel makes an effort to be family-friendly. I've seen a number of families enjoying the place, and the hotel seems to be very accommodating for children.
Enjoy your stay!

The room was simple but clean. The swimming pool was excellent, food not very good and pricey. The highlight of the hotel is that it's very close to the entrance of the fall's park.
I booked the hotel in lastminute.com and the website offers a free shuttle bus from and to the airport. This shuttle bus does not exist at all. The people of reception are unaware of it and not willing to help, in fact the lady in charge of getting the taxis was rude.
I do not recommend this hotel because they haven't heard of customer service. I don't know which fault was it, the hotel or lastminute.com, I just think that the website wouldn't published anything that hadn't been authorized by the hotel.

We took a chance and turned up without a reservation out of season. The place was deserted and we very quickly got R$100 a discount off the room without trying!

The hotel is nice. The rooms are clean but a little basic. Whilst they are away from the restaurant and night club (which was a kids creche when we were there!) the walls are quite thin so you can hear people walking by in the corridor.

The grounds are nice, with a large pool, a big playgound for kids, tennis courts and football pitches and a very nice "Eco walk" around the back of the hotel where you do get to see quite a bit of the wildlife. Breakfast was very nice, being a large buffet with an emphasis on sweet things including cake.

The location is ideal for the bird park which is next door and is much better than it first appears when you walk into it and the Brazillian entrance to the falls which is about a 5 minute walk down the road. However, that is all that there is and if you want to go into Iguacu it's about a 30 minute bus ride (bus picks up right opposite the hotel) or about 15 - 20 minutes in a taxi. There aren't any other bars or restaurants to walk to.

It was deserted as we were out of season but the staff were reasonably friendly and it was very nice and quiet, for the first night it felt like we had the place to ourselves! I can imagine that the place will be packed in high season.

If you do go to Iguacu, make sure that you allow a day to spend on the Argentinian side and that you also go to Puerto de Iguacu. It's a lot smaller, friendlier and cheaper than on the Brazillian town.

After reading the reviews in Trip Advisor I went to the San Martin Hotel with no expectations. I was so wrong.
The hotel is 200-meter away from the Brazilian national park, no shops or restaurants nearby and about 15 minutes from the city of Foz by bus ( which offers good service and the bus stop is just across the street from the hotel)
Following the advise of other guests, we did not stay at the standard room and booked the Junior Suite. ( there are for categories of rooms: standard, superior, superior class and junior suite. both superior class and junior suite are very nice and the superior room is ok as well) The room was spotless, new and modern. Service was quick and efficient, at the reception, pool and restaurant. Breakfast and dinner were buffet, food was fresh and specially for dinner it changed every evening: salads, beef, rice, pasta, and chicken fish or pork, potatoes. Desserts were ok. Love the batido de maracuya! Once a week they offer churrasco for dinner ( barbecue) which was very good.
the pool area is very nice: big pool, big area to sunbathe ( you are given pool towers), lots of shade and a 2.3 km path to exercise. tennis court, football field and playground for kids. In the evenings you can play cards, pool, ping pong or watch a movie.
Next to the hotel you'll find the Birds park which is worth a visit.
My husband and I travelled to Iguazzu thrru an Argentina travel agency, called Ati ,www.ativiajes.com.ar, which owns the hotel and I think you get preferential treatment. They also work with the travel agency desk at the hotel that offers tours in the area. I think that's the weakest point: the guide simply takes you to the falls, etc but does not offer much interisting information.

We had a great relaxing time. totally recomend the place.