What can I do in post-game in Borderlands?

Solution 1:

If you already did everything available, you may want to start New Game+ mode, Playthrough 2:

The character retains all items and levels gained in Playthrough 1 and will face off against similarly higher level enemies in an otherwise identical story. The enemy levels range from level 32 at the start, to level 50 by the end. Items will be encountered in higher level versions to match the enemies and general difficulty of Playthrough 2.

Solution 2:

In addition to playing the game in NG+ I’d also recommend replaying the game with a different character.

Each of the main characters encourages a dramatically different playstyle including stationary support, long range harassment, and tanky fist fighting.

Replaying the game with a new character allows you to flesh out situations and challenges you might not be accustomed to