Although there has been a lot of interest in some corners about the benefits of dual (or even triple) n-back training, much of the research on brain training shows that training across several cognitive domains may be the best way to maintain your brain fitness.
Studies across sites and with different techniques are consistent in showing that multi-domain training is probably the best way to go. That means, first, doing training activities that require both verbal (words) and visual skills. That would mean not only doing something like crossword puzzles (a verbal skill) but also working on doing mazes or a computer-based visual game.
Although the research on cognitive training doesn’t really provide too much guidance, it’s my opinion that it may be a good idea to train on a variety of activities. Why? Given the likelihood that what may stimulate the brain to grow new connections is change, it’s probably important to do something new regularly. Just doing the same activity over and over again may not be the best way to keep your brain fit.