After defeating Whitney, head north to Route 35, where you will encounter a number of trainers. You will see some water that you can't cross just yet, where a Grn Apricorn tree awaits. There is also a grassy area and a bug catching trainer, who stands near a tree that you can Cut. If you cut the tree, you get a shortcut to the northern path here.
If you go into the National Park on a Tuesday, Thursday, or Saturday, it will be the day of the Bug Catching Contest. The Bug Catching contest is a chance to try to catch the best bug pokémon in the park. There are some types of pokémon here that can't be caught anywhere else in HeartGold or SoulSilver. You are given 20 sport balls and are allowed 20 minutes to try to catch the best bug pokémon you can. Your catch is judged by the sum of its stats, as well as by how high its HP are. This means that rarer, higher-level pokémon will get a higher score, especially if their HP are still high.
The Pokéathlon Dome is a place where you can take three of your pokémon into a series of three minigames. Each of your pokémon has special stats that affect its performance in the various Pokéathlon games. You can see these status in the pokémon's summary screen. You will control your pokémon in these games, so it's a combination of your pokémon's stats as well as your practice and skill. The points you earn in the Pokéathlon will accumulate, and you can use them to buy special prizes, including evolutionary stones and other items.
If you haven't already, be sure to get the squirt bottle from the flower shop once you have defeated Whitney.