Buy an Accumate off eBay, you can leave it connected to the battery all the time you're away, it'll keep it in tip-top shape.
Accumates come with two cables. One terminates in a normal set of crocodile clips, the other is fused and terminates in a pair of ring terminals. Putting the car on charge is then simply a case of parking it in the garage, popping the bonnet, plugging the cables from the Accumate into the mains and the cable on the battery, then walking away. You can even drop the bonnet and set the alarm, it'll run the lot quite happily.
Don't buy the closely-related Optimate, they're for bike batteries and can't handle the big boys.