Hi Joyce! Welcome to ATP.
(Edited to add: I see that I cross-posted!)
>> i know it is too late for this season
Depending on your region or climate, it's a good time to start cool-weather crops for late fall harvest. Depending on how hot your summer is, and how early your first fall frosts are, "cool weather crops" could be started around the next few weeks, give or take a month.
Dave has a garden calendar that takes frost dates into account - See "Goodies" on the left-hand menu, then "Garden Planning Calendar", then enter your ZIP code. That would point you towards crops that you could experiment with right now, so you would have more practice in buckets for next spring.
Also, I think that there are kinds of lettuce for summer and kinds for fall.
>> 5-gal pails what kinds of veggies grow in these
You might be surprised! I know I was surprised to learn that you can grow peas (maybe snap peas and snow peas) in pots lower and maybe smaller than 5 gallon buckets. I looked for those threads but haven't found them yet.
@Newyorkrita might be able to suggest some "bucket crops", especially if you clarify where you're growing and whether your climate tends to be hot, dry, cold or whatever. If I were you, I would start with "what would I like most?", and then find out how well those do in buckets.
Lettuce and some Brassica greens like Bok Choy, tatsoi and komatsuna ought to do well in buckets. Swiss Chard is recommended (see links below).
Mostly, I don't know the answer. You might look at a garden calendar and spot some that you like that are listed as fall crops in your region. Then try to find out here or in seed catalogs some varieties of that vegetable that do well in pots.
Our "Plant Database" has a checkbox for "suitable for containers". In fact, I think it lists the size of pots that a plant will do well in (2+ gallon, 3+ gallons, etc.) However, many plants were added without many details being filled out.
I know some people grow some varieties of cherry tomatoes in buckets, and even other tomatoes. It probably IS too late in most areas to start tomatoes this year! You have to look at seed descriptions that include "for containers" or "patio". I often research vegetable varieties in seed catalogs, and you can do that even if you later go plants of the varieties that you choose.
Here are some photos of Newyorkrita's vegetables in pots. I may have listed some here that were only "starts" that would later be planted out in rows.
http://garden.org/thread/view_...
eggplants
celery
yard long beans
Bok Choy
zuchinis (?!?)
melons (?!?)
leeks
http://garden.org/ideas/view/d...
http://garden.org/ideas/view/S...
http://garden.org/ideas/view/S...
Joyce! I don't grow in containers but @newyorkrita does a lot. She might be able to advise you. 
