serebii.net has an extensive explanation of Join Avenue you will find exceedingly useful if you know your stuff.
The Market sells items you can't find in PokeCenter shops, like PP ups and Ethers. Their stock increases as it levels up. The Dojo provides training that increases your Pokemon's EVs by a fixed amount. They also have training that increases a Pokemon's level by several increments at once. (BE WARNED: Pokemon who level in this way will not evolve or learn moves they would normally undergo had you raised them normally. I recommend using this service for EV training hard to train Pokemon like Audino and getting those last few levels to 100) Their services get better as they level up. The Salon has several services that increase your Pokemon's happiness. Their other services decrease Pokemon EVs or erase the EVs completely in a select stat. (Very useful for EV training Pokemon you receive in trades from the GTS) Their services get more potent as they level up. The Deli is a sort of mix between the Dojo and the Salon. They provide EV-increasing, level-increasing, and friendliness-increasing services. Their menu broadens as they level up. The Raffle gives you a 1 in 10 chance of getting a prize. The odds of you getting the top prize increase as they level up. (But it's not really worth it. You'll still have a snowball's chance in Hell regardless) The Antique Shop is an interesting sort of gamble: you pay for an item, and then you find out what's inside it. Things like Nuggets, Ultra Balls, Fossils, and Hard Stones (the most common item you get) are revealed within, allowing you to sell some of it for a small profit. The odds of you getting a better item for your buck increase as it levels up. The Flower Shop sells berries. Their stock increases as they level up. The Nursery (ONLY available after beating the League) offers services that make eggs hatch faster. Their services get better as they level up but who really gives a hoot when you can get better results with your bike and a Pokemon with Flame Body/Magma Armor? Don't waste your space with it.
If you don't know a thing about EVs, check marriland.com. It's not as tiresome as it seems.
serebii also tells you what visitors make what shops and what messages recommend them to what shops. As you grow in popularity, you get more visitors. Gym Leaders especially boost a shop's rank by 1000 points (yowza) so make sure you catch them if they pass through!
Join Avenue is an awesome addition. In the early part of the game I suggest having a cafe, arena, and massage parlor, as all boost stats, can raise levels and (on exception of arenas). Both the avenue and shops can level up. The avenue opens up customization and better shop goods on level up. Shops improve wares and change appearance every 4 levels. This is done by reccommeding passing people once a day in the avenue and recommending shops. This is how both shops and avenue level up.