Manny:
As a LEO here in the great melting pot (and retirement community) of Florida, I would have to agree with several of the above users. My suggestion as a guy who continually deals with these EXACT type homes everyday are as follows:
1.) Buy and place a double motion light at front right and rear left corner of the residence. Point the light down one side of the house and the other down the other side. Once you have installed both double motion lights (depending on the size and shape of the house) you should have total coverage. I would also buy and place one above the garage door pointing in the driveway or at their vehicles. Although not a TOTAL deterant to a subject thinking about committing a crime, it makes them think twice about snooping around in very well lit areas. It can also alert your parents when something is moving around the house.
2.) Make sure as Japle said, don't go cheap with finding a way to secure the sliders. 85% or burgalies down here occur when the subject(s) make entry through a slider or "french type" doors. It is very VERY simple to pop open a slider simply by lifting up and laying it down. My suggestion is the same way I secure mine. Make sure they are using the "inside" slider door...that way you can place a large 2x2 or even steel pole (cut to length) between the rear of the slider and the slider door frame. Dont' be fooled, the little crappy lock that secures your slider is easily defeated with a common flat head screw driver. I also use a metal dowel pin that I screwed through BOTH doors to prevent the door from being lifted up and "popped".
3.) When your parents are not home, I would also suggest that they close their blinds or curtains to prevent anyone from looking inside. If they can't stand closing the blinds, at a minumum make sure they do not leave valuables that tempt burglars to steal in the first place. Perfect example, I answered an alarm call the other day. Upon arrival, the front "french" doors were wide open. Of course my zone partner and I cleared the house and it seems as though it was a bad lock up...however, every blind and curtain were open. The home owner had jewelry on the table paired with about $300 in twenties. They also many other valuables just laying around. It would have been very easy for someone to see the door wasn't secured when the homeowner left and take advantage of the goodies inside.
4.) As far as the screen lanai or "pool deck" is concerned, they are only designed to prevent trash from blowing in the pool and a tad bit of shade and nothing else. They are not made of steel and screen is just that....a screen. The tiny cheap lock is easily defeated when you cut a small hole beneath the door handle and reach in unlocking it by hand. Don't be fooled to think a locked pool deck will keep out a criminal....ALWAYS lock the doors leading into the home from the pool deck when your not home...ALWAYS.
5.) Camera systems are great and all, but they are expensive and they do required continuous maintainance and attention. They are great for catching the burglar on film, however...unless the burglar is known to the home owner or is wearing a picture of a license on his t-shirt...he will only be identified as a w/m, b/m, w/f, b/f, with said clothing if even that. Every now and then a camera does help solve a case...so they can be effective but costly. Fake cameras are also a smart idea. But as we see from the evening news everynight, it doesn't stop the handy way robbers from robbing that store or that bank.
6.) Honestly, in my entire 6 years of being a LEO I have never responded to a burglary where the home owner owned a large/medium sized dog. Dogs IMHO are the best way to make sure your home is never broken into. They'll alert you when your home or sleeping and make a burglar think twice about trying to go inside when your not there. However, as you advised earlier, they can't have a dog because of allergy issues...so, the next best thing is a ridiculously LOUD siren. Make sure when it is installed that it is installed at both the front, rear (exterior) AND interior of the residence...sirens are cheap and easy to install. Also make sure that any exterior siren you install is installed inside the soffit just below the roof. That way, the wires cannot be cut and the siren cannot be seen...and it will still be loud enough to wake the dead. You'd be suprised how many alarms we respond to that were called in by neighbors or passer bys because it was audible. In many instances, we have had calls where a burglar made forced entry into the home...then scrammed when the audible alarm sounded...or got a lot less items than he orginally anticipated.
I could honestly go on all day about what helps. Point is, is that if someone is really intent of burglarizing or making forced entry into your home...their gonna do it. And as far as "home invasions" go, they seriously rarely occur (home invasion meaning burglars entering your home with people inside). And usually when they do occur, it's scumbag on scumbag or someone who has a vendetta..or owes money...etc. This may all seem like alot, but when you make it part of your routine, you will not even notice. I practice these things daily at my own home...even with a P-car in my driveway. Criminals are criminals because they are inheirently stupid...thats why they are criminals. But just a few precautions and you can bring your chance of your home being targeted by huge amounts. I am not an expert and do not EVER claim to be. I am simply stating my observations based on my own personal experiences..take it for what it is. I'm sure someone will have something smart to say...but hey, what else are forums for?