A great weapon to have in your arsenal is a solid service return. You can use the return to put your opponent on the defensive at the first strike of the ball. If you receive a weak second serve, it gives you the opportunity to be aggressive and hit out on your return.
A weak second serve, or first serve for that matter, is one that doesn't have much pace or spin and lands in the middle of the service court, or shorter. It is one that you can easily step in and maneuver around to hit your favorite shot. Because the pace is slower, you should have time to get your racquet back early and prepare mentally to execute the stroke correctly. This is what gives you the advantage.
If you like to hit hard, and like to play big hitters, a weak second serve is something you should be able to take advantage of. Even if you don't hit with pace, you should still find it possible to handle a weak serve and return it deep into the court. This doesn't mean you have to hit it for a winner. A solid return that goes deep will set up the next shot that you may have an opportunity to put away.
It is sometimes difficult to adjust your game to someone who doesn't put much pace on the ball, because you have to generate it all yourself. To be successful against this type of player, you will need to develop patience and put a bit more spin on your returns. There's no reason why you can't blast the weak second serve back for a winner, but you may not always be in a position to do so. It's most important to keep the ball in play and return it deep, if possible. If you're not able to control the shot, add a bit more topspin and wait for a better opportunity.
Practice with a partner who hits slow or medium speed balls, and work on driving your return for consistency. When you can hit 7 of 10 deep in the court, go for a bit more power, and make sure you are always in control of your swing. Of course, practice makes perfect. You should allow yourself to "go for it" at certain times in a match, but should generally try to be ahead in the game when you do. If you miss the return, go back to just getting it in play until your timing improves.
Remember that just because your opponent is hitting a weak serve doesn't automatically mean you an hit a winner off it. You need to learn to wait for the right shot.
Good luck on the courts!