I can only second what people have had to say about Maine Road FC so far. Though attendences are low there's a great community spirit, and though budgets are small for the level they attract exciting players and play attractive football.
I first went to watch them simply because they were local to where i was living, but i must admit i was quite taken with the name! I still go and see them when possible, but like others have said, it depends on what City are doing. If we're playing then it's a simple decision, but with kick-off times nowadays Saturday games can be either side of a 3pm Road fixture, and now we're back in the Europa League we'll mainly be playing on Sundays. Midweek fixtures in their division are on a Tuesday, too, which is handy.
Personally, i just like to see a game of football, and if it's not City it doesn't really matter whether it's Chelsea or Bolton Vets. There are a lot of good footballers at Vodkat League level, and plenty of banter in the stands. With many of the people at Maine Road being Blues, as well, it's an extra opportunity to talk City. The pies are bloody good, too!
I'd try and encourage anyone to get down to Brantingham Road. You don't have to go regularly, just give it a whirl, remember what it's like to get home on a Saturday having been fed, watered and seen a good game and still have change from £20! If not Road then whoever your local side might be. Saves you getting dragged to the Trafford Centre at least, and if you're interested most clubs will appreciate any extra pairs of hands, so it could be a chance to get involved at the grass roots of the game.