After going east through Route 121 where the Safari Zone is, you reach Lilycove City.

Megamart

There is a Megamart in this city, but before you can enter it, you have to battle your neighbor.

In Ruby and Sapphire, if you chose Treecko, your neighbor has a level 31 Swellow, level 32 Wailmer, level 32 Shroomish, and level 34 Combusken. If you chose Torchic, your neighbor has a level 31 Swellow, level 32 Shroomish, level 32 Numel, and level 34 Marshtomp. If you chose Mudkip, your neighbor has a level 31 Swellow, a level 32 Numel, a level 32 Wailmer, and a level 34 Grovyle.

In Emerald, if you chose Treecko, your neighbor has a level 31 Tropius, level 32 Ludicolo, level 32 Pelipper, and level 34 Combusken. If you chose Torchic, your neighbor has a level 31 Tropius, level 32 Ludicolo, level 32 Slugma, and level 34 Marshtomp. If you chose Mudkip, your neighbor has a level 31 Tropius, level 32 Pelipper, level 32 Slugma, and level 34 Grovyle.

Get Items

You can get a berry from a Gentleman outside, and get TM44 Rest nearby. You can also get a Max Repel in the southeast part of town. The beach has a hidden Heart Scale on a rock.

Explore More

In Lilycove, there is a museum. The Move Deleter is in this city. There is a Contest Hall here.

Moving On

You have to go east from Lilycove, but Team Aqua/Magma is blocking the way. The only thing you can do now is go back to Route 121, then use the dock in the middle to Surf south to Route 122.