A very nice source of practice for beginning readers of Japanese is the 'Watanoshii Free Web Magazine in Simple Japanese':
It has hundreds of articles with mouse-over word translations and full text translations available. Some articles also have good quality audio as well. Highly recommended for JLPT N5/N4 learners!
For not just learning about grammar, but also getting some practice putting together sentences, the Wasabi website has some nice online activities!
For each grammar point there are a bunch of sentences given for you to translate, and you can tap the answer field to see the answer (works fine on smartphones too).
You can also do this activity aurally, playing a soundfile to hear the prompts and answers.
Try it out here:
Oliver Rose in Kansai, Japan