Head across the river yet again to the West and you can catch up to Cheren, who is waiting for you outside of a curious little building.  He asks you to come with hem and leads you inside.  There are a lot of pokemon and some scientists here, and Cheren begins to explain.

It's not well-known, but apparently Lacunosa Town experienced a strange sudden drop in temperature.  The drop was brief, but it went below the freezing point of water.  Apparently, similar incidents happened near Castelia and Driftveil.  The Gym Leader believes that these incidents may be connected with Team Plasma- which could be a very bad sign.

For whatever reason, this brings up for him that he hates cold weather, and so he gives you the HM for Surf (what).

At this point, having Surf and Strength both available to you, it would be a good idea to head back to the beginning of the region and go over all the areas you might have missed because you couldn't reach them without the two HM moves.  Before you leave, though, you may want to talk to a scientist in the back of the lab, in front of the four cases with different-season plants in them.  He offers you a special Deerling

A quick Fly to Aspertia will bring you back where you started- the front door of your house.  Across the pond there you can find the TM for Energy Ball, TM53.  Energy Ball is a fairly strong Grass-type special attack that stands a chance of lowering the target's Special Defense if it hits, much like Flash Cannon or Psychic.  While very few non-Grass pokemon can learn it, it's a very handy attack for most Grass types, who otherwise have to rely on Giga Drain (less powerful, though the HP drain is nice) and Solarbeam (charges for a turn before each attack) for a reliable high-power Grass attack.

North of Aspertia on Route 19, you can find another small lake near the Trainer Tips billboard.  Surf North onto it and head West, then North again to grab a Water Stone.  Turn and go East on the water and you can find a stairway, at the top of which is a Nest Ball.  Head South on the ridge to find the spot Alder likes to leap off of, and an Expert Belt hidden in the grass.