By bus
Adventurous types can try the bus system to reach those areas not covered by the subway. There are a few resources that help english speakers navigate, but it helps if you know the city or at least some Japanese, as most stops has route maps. In this case it might become very convenient and far-reaching service, as many points in the city are reachable only by bus or car. However, in some places buses are infrequent, with waiting times up to half an hour, and they tend to get very crowded in the rush hour.
Fortunately for tourists there is a "Loople Sendai" (http://www.kotsu.city.sen...) bus that makes a wide loop around to various Sendai attractions for ¥600 for a whole day, or ¥250 for one ride. Normal route buses have fares based on the distance travelled and usually cost from ¥170 the basic fare to ¥300 for some longest journeys.
Travelcards commony called "bus cards" both in English and Japanese, although they could be also used on subway are available, offering slight discount -- ¥5,000 card has ¥5,850-worth of fare on it. There are two major bus companies, but for all practical intents and purposes they are indistinguishable.
On foot
The city center is compact and can easily be traversed on foot, especially by using the covered shopping arcades. There are many shops and arcades around Sendai station and therefore people could walk around on their own. Other parts of the city are quite hilly even the center has some significant slopes and while they still could be traversed on foot, this might be physically demanding. Residential parts are also very spread-out, and walking such large distances becomes impractical.
By Subway
Sendai has one subway line traveling on a north-south axis, connecting major shopping districts with the train station. Key stations include Sendai for the train station and the AER building, Nagamachi-minami for the Mall large American-style shopping mall including multi-screen cinema, Hirose-dori and Kotodai-koen for access to Ichibancho covered shopping arcade, and Izumi-chuo for the soccer stadium. Second east-west line is under construction, with opening planned in 2015.