I've been teaching a group of adults in my village for about 6 months now and we have une baleine of a time! They are very mixed ability from the mayor, who's over 70 and finds it hard (but is game for a laugh and is not afraid to have a go) to a neighbour whose wife is Swedish and is pretty fluent.
I use a mixture of resources - we use Harrap's Anglais Méthode Integrale CD and book (Available on Amazon.fr) by Sandra Stevens. You can buy 1 book+CD pack and then furthercopies of the book separately for the students (around 7,50 € for the book on its own). I like this because it's very practical (it starts with asking for food and drinks on a plane) but from the word go focuses on pronunciation which even the more competent students find challenging. It's also good because all the explanations are in French.
I back this up with a brilliant old book called "Essential Grammar in Use" by Raymond Murphy which is probably out of print but can be bought for a song on www.abebooks.com This has very good explanations (in English) of a very wide range of English grammar points, with examples and exercises. This is very good for mixed classes because for any particular topic the beginners can focus on the first, most simple, exercise while the more advanced students can steam ahead with the whole page.
I always finish the class with something light - ideas like copying pages from English recipe books, cutting up the instructions and getting them to put them back together again (working pairs allows the advanced ones to help the beginners). We also do role plays and things like shopping games. Bingo is very popular and we use word bingo to reinforce vocabulary. This morning I discovered www.eslactivities.com which allows you to create custom bingo cards from word lists you create yourself, so this evening we're doing a food and drink bingo with the words we've done over the past few months.