There is more to the Lostlorn Forest, but the rest is only accessible much later in the game, when you have several more HMs at your disposal.

Returning to Route 16, continue east to face a pair of trainers in a double battle.

Battle: Backers Stu & Art (1152 Poke)
Opponent: Scraggy Lv 24
Opponent: Scraggy Lv 24

And then a bicyclist, who sometimes follows the road to the East and then up North a short distance.

Battle: Cyclist Krissa (1060 Poke)
Opponent: Whirlipede Lv 25

Past Krissa you’ll find the gate to the Marvelous Bridge, where you can gather a Fresh Water from a man who got two out of the vending machine and wants to share.  The elevator to the bridge is busted, though, and you can’t get on- so your exploration in this direction is already done for now.

Return to Nimbasa City (using the Pokemon Center there if you need to) and scope out the East side- the theme park area is still there.  Head South from the entrance and grab a Dropped Item before going on- it appears to be someone’s lost Xtransceiver.  As soon as you grab it, it rings, and the person who owned it informs you that they lost it and are calling you on an old model (hence the audio only).  They want their Xtransceiver back, but don’t have time to pick it up now- so they tell you to hold on to it for a while.... and identify themself as ‘Chr... ah, I mean, Curtis!’  The person says they’ll call you regularly on your Xtransceiver to keep you updated so they can pick up the thing when whatever work they’re doing dies down enough.