If you travel east along the northern border of Solaceon Town, you will see three sets of ledges to jump down. Take the middle one, then jump down the next two and you will arrive at the house of the seal woman who will give you the Seal Case, and her son, who will hand over an alphabet Seal for each corresponding Unown you show him (including ? and !).
Most notably in Solaceon Town is the Pokémon Day Care in the northwest corner of the town. You can deposit up to two Pokémon that you want to be trained by talking to the Day-Care Lady. It will cost 100 as a starting fee and 100 more for each level the Pokémon gains. The Pokémon deposited gain one exp. point per step you walk, however they won't grow to be as strong as Pokémon raised to an equivalent level solely through battling. When you deposit a Pokémon into the Day Care for the first time, a man will appear at the table inside. When spoken to, he will give you the Pokétch app Day Care Checker. This is where you can breed Pokémon by depositing two of the same Egg Group.
East of the town are the Solaceon Ruins. You will be able to catch all 28 forms of Unown here, and this is also the location of the HM for Defog. Right now, you will only be able to find 26 of these forms and by catching them, the Maniac Tunnel on Route 214 will expand. When you have collected all 26, you can travel through the Maniac Tunnel to access the hidden second floor where the last two Unown forms (? and !) are. Make sure to encounter at least one wild Unown since no Trainer uses an Unown and you need it to complete your Sinnoh Dex.
Twenty of the Unown can be found in the dead-end rooms, and since not all of them can be randomly found in the same room, you will have to do some exploring. The remaining six forms are located in all the main hallways, excluding the entrance, and only one form can be found in each hallway.
If you have trouble reading the inscription at the north end of the entrance hallway, it says: Top right Lower left Top right Top left Top left Lower left
Follow the corresponding stairways to find a room with HM05 (Defog), the Mind Plate, and the Odd Incense (for producing a Mime Jr. egg when breeding a Mr. Mime). Once Defog is in your possession, your mission here should be complete. A Fire Stone, Water Stone, and Thunderstone are hidden in the boulders in three of the dead end rooms.
The Lost Tower is a five-floor building on Route 209. While the first two floors are easy to navigate, on the last three you will need HM05 to remove the thick fog. If you come here during the night, you can find wild Duskull. Once you reach 5F, talk to both of the elderly ladies to receive the Cleanse Tag and Spell Tag.
Once you've left the Lost Tower, return north to Solaceon Town and heal up for the journey ahead, then go north to Route 210.
Head into the left hand patch of grass on Route 210 and up the western muddy ramp, and continue north past the Twins and jump over the ledge. Head back south along the path between the two grassy areas, east along the hill and then north to find a woman looking out over a Honey Tree and the Cafe Cabin. Speak to her, and she'll hand over TM51 (Roost). Backtrack to the top of the path between the grass, and enter the Café Cabin.
Trainers with a Vs. Seeker by their names, when alerted for a rematch using the item, may use higher-level Pokémon.
In the Café Cabin, there are a number of Trainers to battle, and you can buy one bottle of Moomoo Milk (which restores a Pokémon's HP by 100 points) for 500 or 6000 for a case of 12. Since a group of Psyduck are blocking access to the northern section of Route 210, head east instead to Route 215 instead.
Route 215 is packed with Trainers and items, and the constant rainfall boosts the power of Water-type moves by 50% (and decreases the power of Fire-type moves by 50%), so this is a good place to train Water-type Pokémon. Bring a Pokémon that knows Cut - it will make travel significantly easier and will allow you to collect all the items on your way to Veilstone City. Head east a short distance, then circle around anticlockwise up and across the first bridge. Follow the path until you reach the third bridge, make a detour to fight the Black Belt to its south, then Cut your way through to another Black Belt to collect the Fist Plate and jump over the ledge. Use Cut on the tree below the next hill to harvest some Berries, then head up the hill, detour south across the fourth and final bridge. Follow the path onward collecting all the items that are of interest, making sure to Cut your way through to TM34 (Shock Wave) just below the largest grassy patch.