Here it is a rainy Saturday afternoon, and there's not much to do. Going to the movies doesn't seem like such a good idea, unless you're dying to see Crocodile Dundee Got Fingered or whatever. So how about this? Ride the Tri-Rail. If you live down south, hop on at the Hollywood station and locomote through Fort Lauderdale, Pompano and Deerfield beaches, then Boca and Delray, to the end of the line in Mangonia Park. If you live up north, take it the other way. Or try both. Flat-fare, all-day adult tickets are available on weekends for $4, and they're good for one-way, round-trip, or multiple rides in each direction. So pick up your portable CD player, grab your favorite book, and enjoy. Or look out the window and be glad you're not stuck in the traffic jam that's bound to spring up on I-95.