You need drainage holes! Even really experienced gardeners have a hard time getting watering just right without drainage. Put a tray or something under your container to protect the furniture. Better yet, if you use something that will hold a little water (1/4 to 1/2"), you can water your seedlings from the bottom once they germinate (put water in the tray and let it get absorbed through the drainage holes). When the surface of the soil stays wet, you can get a lot of issues -- fungus gnats, damping off, etc.
Pansies and lettuce can take a frost. Pansies can even make it through the winter in many areas. They may look bad after a hard freeze, but they'll bounce right back.
"Cut and come again" lettuce sounds like just the thing to get your sister more enthused about container gardening on the back porch! Keep picking the larger outer leaves, and the plants will produce for a long time.
Lavender can survive a summer in a 1 gallon pot but isn't happy that way. (I had a "leftover" plant spend most of the summer on the deck in its nursery pot.) I am not sure it would do well in a railing planter, as most are fairly shallow. I did leave a rosemary plant in a fairly deep railing planter since last spring -- looks like it made it through the winter, but it's still fairly small, so I'll be moving it elsewhere asap.
Thyme and oregano do just fine in railing planters. If I grew nothing else edible, I'd grow fresh herbs, especially oregano and basil. (I love thyme but get annoyed trying to harvest it in larger quantities.) The price of fresh herbs in the grocery store is out of sight!
Pansies are edible, btw. I find the blue ones often have the best flavor, not sure why. Other colors may not taste like much, but they're still pretty in a salad. Nasturtiums are peppery!
