Hoa Binh city is the Provincial capital of Hoa Binh province and arguably the only major city you will go past if travelling to Mai Chau village. It has all the amenities that you would expect a major town to have though the electricity is not always reliable.
Simply leave Hanoi by taking the road going South from the Temple of Literature (Ton Duc Tang) And follow it turns into Highway 6. Keep following this Highway until you see a road directing you towards the city. The highway itself avoids entering the city.
By Bus from Hanoi
The majority of buses to Hoa Binh depart from My Dinh Bus station in Hanoi. It may also be possible to find buses going in that direction at other bus stations. There is no need to book a bus with a tour company unless you are travelling with a large group. The bus should cost around 35,000 VND and many of the buses also continue onward to Mai Chau.
The city can be accessed by foot though using the local "Xe Om" service or your own motorbike will make things much easier.
Hoa Binh Dam is worth a look and there are also tours available and tickets can be purchased at the main entrance.
For those who enjoy looking at Soviet architecture there is a whole district of flat blocks that was built by the Russians on Hoa Binh street on the right side of the river. It comes complete with some Russian style flat blocks, a school and various other things. It is in some ways like walking round a soviet style town.
There is also a small museum in the town with some prehistoric human remains.
Opening Hours: 07:30 to 11:00 and 14:00 to 17:00
Boat tours are available on Hoa Binh lake nearby and some of the islands on the lake can be accessed. These tours though are not easily available to a non Vietnamese speaking person and would require local assistance.
The city has many shops and a market. There is also a supermarket in the newer district of the town with a reasonable supply of western products.
On the left side of the river is the older part of the town. Eateries are generally what you would expect from a standard city in Vietnam. (Pho, Banh Cuon Bia Hoi, Dog meat etc.) On the other side of the river away from Highway 6 is the newer part of the city where there are some nicer restaurants with full menus and a wider choice of drinks.
There is a nightclub next to V star resort (see below) Otherwise on the right side of the river there is a surprising number of cocktail bars and restaurants to enjoy. Prices here are also much cheaper than Hanoi.
V Star resort (Tổ 18 - Phường Hữu Nghị - TP.Hòa Bình Tel. 0218.3888.688) A complete resort with a swimming pool, bar, restaurant and even a night club. This place was probably built by the Russians and is starting to look a little old but is still a reasonable place to stop off. Be warned though that it is unlikely anyone will be able to speak much English. Also electricity might not always be on as the hotel doesn't have backup generators for the rooms. Cost should range between $20 - $40 depending on the room size and standard, season and ability of the traveler to negotiate (or you could just book online). 
If this is too expensive there is also a cheaper Motel right next to the resort.
After enjoying Hoa Binh you can continue onwards to Mai Chau by bus or motorbike. Simply keep following Highway 6. It is also possible to go to the more remote town of Da Bac following a smaller route Northwest. The town has a 1 or 2 small motels and a pleasant mountainous climate and would be a good place to stay to see more 'non touristy' regions of Vietnam. The road is asphalt all the way there.