Puzzles
Puzzles are fun and a great way to practice your tactical skill - to look for and find opportunities on the board. Historically, they were often of the 'Mate in 2' variety, which did not relate well to real games. However, with the advent of online chess, computers now produce puzzles from past games played online. These are more varied in style and could be find the mate / win a piece / avoid losing a piece / avoid mate - but you are only asked to find the best move, not told which variety. This is much nearer what you are faced with in a real game. However, puzzles do not help your openings or positional play.
Here are the daily puzzles from Shredder chess, one each for easy / medium / hard. Just drag and drop the piece you want to move. There is a hint button, if you need it, and more puzzles to be found by clicking on 'more'!
For many more puzzles, here is a link to Lichess Puzzles. You can set the level between easy and hard, but if you register with Lichess, the puzzles are tailored to your ability. If you keep failing, the puzzles are made easier, but if you keep succeeding, the puzzles are made harder! You are also given a rating and a chart of your progress over time.