There is an abiding image both in Japan and abroad that you need to travel to the mountainous regions of the country to experience the hot springs. In fact, while regional Japan does have some of the most famous and photographed onsen, there are many traditional establishments located in and near Tokyo that offer an equally good glimpse into the ubiquitous culture of public bathing.
One common question is regarding tattoo policy at the day onsen. Unfortunately, many establishments have a strict policy of denying entry to anyone with body markings, but if the tattoo isn’t too conspicuous you should be able to use the onsen if you cover it up. See here for more details.