In my campaign, my players each have several different characters at the home base. Every session, they pick which one they want to play. However, some players always use the same one and so have more powerful characters, while others have three or four characters they use. Also, players regularly create new characters (which are at level 1). The system is AD&D 2e.
Is there a way to shape dungeons, wildernesses and random encounters so they are more applicable to a range of levels? I know that using a lot of weak enemies means everyone can contribute different amounts, but I don't really have any other ideas apart from that. Mostly, I want to be able to set up the sessions so that most of the players are active and at least feel useful. In previous games I've participated in with level disparities, the stronger characters consistently outdid the weaker ones (as you'd expect), but the players of the weaker characters were fairly bored - they knew that they weren't going to affect the encounters or obstacles as much as the stronger players, so they were less engaged.
I mostly need a way to set up combat so that the high-level characters get enough XP that they can level up after a few sessions, while the lower levels are able to catch up after 3-4 sessions. A Dwarven Level 5/4 Cleric/Fighter is the strongest so far, but down the line the disparity could go up to 12 levels or more, depending on how successful the campaign is.
I specifically want to avoid any suggestions about changing the levels of starting players, or giving different players different amounts of XP.
To summarise: How do I set up encounters/dungeons so that lower and higher level players can be active and achieve?