ANSWER - This is the kind of question I love getting because it really makes me put my thinking cap on. What we’re looking for here is something that you can identify as an altar but something that anybody lacking knowledge of witchcraft would entirely overlook. A friend of mine had a brilliant idea the other day (inspired by my candles burning a huge hole in the curtains) – she suggested that instead of using physical depictions of the elements in a circle, instead a witch could use mythical creature representations of the elements. And I think this would be fabulous for an altar for you. A small statue of a dragon (fire), a tree (earth), a fairy (air) and a mermaid/fish/sea horse (water) would go completely unremarked upon in a house but you will understand the symbolism behind each of the statues and so to you it will have a proper significance as an altar.
The other important thing about an altar is that you want something that is unlikely to be handled. Again, statues work well because you can always ask kids/guests not to touch your breakables. But you could also try doing something similar with herbs on a high kitchen shelf. This has the advantage that herbs are so “boring” none of your kinds will ever dismantle your altar. Again, you could use different representations for the elements – chilli for fire, nutmeg for earth – let your imagination run wild, lol.
In your personal situation the best thing to do is to hide in plain view. Instead of hiding things in secret, you need to be very very open with your private things but hide their relevance and significance to you. By having a disguised altar you have all the benefits of following your personal faith but the comfort of knowing that you will never have to explain your beliefs or practises to others. It may also be worth doing some secrecy spell work on the altar to diminish its appeal to others and reduce its interest. A simple shield should work well for this.
(Another quick and simple way is to use photos - even a simple holiday snap of the beach could represent water on your altar..)
Image http://www.sxc.hu/browse.phtml?f=view&id=1423223 (leonardobc)