This simple and convenient lexicon will give you quick substitutes for over 16,000 most popular Spanish words. It does not provide lexical information about the source word nor the synonyms, though, so depending on your level of knowledge in Spanish it may be of more or less use.
Make sure you download and test the evaluation copy before purchasing the full version. The downloadable trial comes with the Public version of BDicty Dictionary Reader and the lexicon itself contains only Spanish words that start with "A", "B" and "C".
The full version contains 16,300 words with over 60,000 synonyms and occupies 630 kB. It can be placed in the main memory or on external media card.