Any suggestions on day trips by train from Lisbon?
I suggest Sintra, Cascais and Estoril. Enjoy!
In addition to those in the above post (all detailed in Rick's Portugal book), I liked Santarem (an hour from Lisbon) and Tomar (two hours from Lisbon, but on the same train line as Santarem). These are not in Rick's book; I used Lonely Planet and Rough Guide for them.
I definitely agree with the Sintra suggestion. It's easy by train and there are more things to see than can be accomplished in one day! Of course, I think Cascais is worth it - but is admittedly much better in warm, sunny weather. You can also take a ferry across the Tagus river to get wonderful views of Lisbon.
Thanks you all for your wonderful input.