Ask Mikey Styles for advice

and then do the opposite
Seriously, when you say "letting family think I'm going some where else", are you talking about family as in brothers, sisters, surviving parents and maybe K*ds from a previous marriage or are you really saying WIFE? It makes a difference. If you're a single guy and/or you're only talking about work colleagues and extended family, then the-less-said-the-better/don't-ask-don't-tell approach probably works well enough. A vague but plausible cover story will probably suffice. Some might strongly suspect, but who really cares? You have plausible deniability and no one is going to care that much to investigate hard enough to find you out. OTOH, if you're saying "letting MY WIFE think I'm going some where else" then you're surfing in dangerous waters. You can never overestimate the degree of sniffing around a suspicious wife might do and, if you're taking multiple trips to CR or vaguely specified trips anywhere else, eventually your wife is going to start to suspect if she hasn't already.
Now, as to cover stories, there are basically 2 ways to go. You can make up a reason why you keep going to as often as you do or you can pretend to be going to some other place (or places).
You have obviously elected the latter approach. This approach requires the use of Skype or something similar IF you have any plans to call your home or office while you're away. It also requires you to cover your tracks in terms of any financial records that might come back to you IF you're married and your wife has access to those records. Of course, you could always just pay cash for everything for that, but most people at least put their hotel on the ol' CC when they travel and paying cash for plane tickets raises a whole host of issues these days. The Alibi Network can take care of all these things for you, not just the phone issue. Of course, they can be very expensive but the CRT VIP discount can help at least a little bit with that and they're still far cheaper than a divorce.
Unfortunately, I don't think even the AlibiNetwork can do anything about the stamps in your passport, so you'll need to keep that in a place your wife can't get to. Ideally, she shouldn't even know you have one and for guys just now considering their first trip to CR and applying for a new passport, it may be possible to keep all that secret from your wife and everyone else. Unfortunately, most of us are probably well beyond that point now if the people around us already know we've taken foreign trips. And if a wife suspects you've been lying about where you've been going she is eventually going to ask to see your passport or try to dig it up on her own.
Some other thoughts for you married guys. Obviously, you don't want them to see you packing pocket-rockets and victoria secret panty gift sets and a box of 3 dozen condoms. But you also don't want them to see you packing all casualwear when you say you're going on a business trip or warm-weather gear when you're saying you're going to a cold-weather clime. One way around that is to pack 2 seperate bags - the one she sees you pack at home and put in your car trunk when you leave and the one you hide somewhere else ready to switch before you go to the airport and get on the plane. The drawback here is: a) that is just one more lie she can catch you in and b) your wife may want to drive you to the airport (more on that in a moment). A better way around it is to choose a comparable destination and trip purpose for your cover story so that you don't HAVE to pack different bags. If your wife drives you to the airport or picks you up (which she might insist on if she's suspicious), pick a fake destination with flights roughly the same time as yours preferably on the same airline as the one you're actually using and PRAY that she doesn't ask to see the ticket or that your flight gets in late when she goes to pick you up.
If you really think about it, married guys can really go crazy thinking up ways they could easily get tripped up with lies like this. Coming up with ways to explain your trips to CR is hard enough. Coming up with ways to explain why you've been lying about going somewhere else, which could easily happen is even worse. Because of this reason alone, I don't think lying to your wife about where you're going is really a good way to go (unless she doesn't even know about your passport). OTOH, making up an alternate place cover story might suffice for friends and colleagues, who are less likely to be as inquisitive.
That still leaves you with the issue of explaining why you're going where ever it is you're saying you're going. You have taken the-less-said-the-better approach on that because, as you so astutely noted, the more you say the more likely you'll eventually contradict yourself. Unfortunately, no matter which approach you take (claiming to go elsewhere or admitting to going repeatedly to CR), you still have to be prepared eventually to answer a few questions or maybe provide a few pics if nothing else. This can still be done if you say you're going somewhere else, particularly if you've actually been to that place before or know enough about it to answer the level of questions you might possibly encounter. However, I have found it is always easiest to lie when the lie contains an element of truth (e.g. not having to hide your passport or flight itinerary or explain why you were lying about that if they're discovered). A number of posters have already provided a variety of explanations that one could use to explain multiple visits to CR. It could also help your cover story if you actually spend at least part of your trip actually doing what you say you were doing the whole trip. One of my favorite tricks (not that I need to do this stuff myself) are reported by guys who go golfing or fishing ONE day but bring along 4-5 different shirts that they can change into and take pictures of themselves in (posing with fish or standing on different greens)in order to APPEAR like they did that activity on MULTIPLE days. If you say you are property shopping then download some RE pics to show your friends to "get their opinions on". You can also easily get pics taken by others of various remote places in CR. They won't have YOU in any of them but you can still claim them as your own to "show off the wonderful eco-trip you just got back from" but really to "prove" you actually spent your time exploring the country like you maintain.