Answer: There are quite a few tropical looking plants that will thrive in zones 6 and 7. Think first about banana. Your growing season isn't long enough for banana plants to produce actual bananas. Nevertheless, the plants themselves are hardy and will come back each year and multiply. You need to look for a variety called Basjoo.
You might also consider palms. The hardiest is the needle palm. It is short, shrubby and slow growing but is hardy to zero or below. The larger and faster growing palm is the chinese windmill palm or Trachycarpus. The tree itself is hardy to around zero although it will lose most of its leaves at this temperature. This is a single trunked palm that grows rapidly and in a few years will be a couple of feet high. For best success with this palm it is best to protect it in winter by wrapping it. It's worth the effort.
Other plants you can grow are various gingers, cannas, especially the variegated leaved forms. Various elephant ears (alocasia, colocasia and xanthosoma) are a must for the tropical garden and while not hardy, can be overwintered indoors. There are many tropical plants that you can plant directly in the ground and then take cuttings in the fall or treat them as annuals and buy new ones every year. For instance, inexpensive hanging baskets of wandering jew make an excellent fast growing, colorful ground cover in the shade. Philodendrons, spider plants and pothos are other options. Be creative.
For a lush tropical look you need to plant thickly. A plant here and there will not give the look and feel you want to achieve. Also, keep your daisies and hollyhocks out of your tropical garden; they just don't work well together and they'll take away from tropical look. For tropical gardening you'll want to plant in the warmest possible location in your yard. This is usually up against a warm south wall of your house or a protected court yard. The richer your soil the better. Tropicals also demand a lot of water so be prepared to water deeply in times of drought. Two books you might want to buy or get from your library on the subject are "Hot Plants For Cool Climates" by Dennis Schrader and "The Exotic Garden" by Richard Iverson. Some mailorder sites for tropicals are: http://www.plantdelights.com, brentandbeckysbulbs.com and don't overlook ebay. You'll find lots of inexpensive things here.
Hope this gets you started on your dream garden!
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