Become an enabler.
Enable other players,
Sit in the back playing the strong silent type and use your prior knowledge to enable other players. Be silent Bob, be a support character, don't say anything until you absolutely need to, but sit back and subtly prepare yourself to help other players make the most of a situation, you know a big encounter is coming don't prepare fireball, prepare bless and haste, help other players take the spotlight. Be the guy that reminds other players of their abilities or uses your abilities to help them shine. Be the guy who charges in just to give the rogue the ability to sneak attack.
This is a good time to pull out the fish out of water character, the newly freed slave, the just awaken war forged, the tribal hunter who got arrested for sleeping in the street because it is his first time in a city, the cloistered cleric who has not left the monastery since they were 12, the fatalistic barbarian who does not fear death just death out of combat, character who would have no idea what is going on and act in unusual manners. Actually a cleric or druid is great choice as you can use whispers from the wild or communing with your god as sources for your "hints". It helps if you play a character who morals/priorities are a little weird so your goals and everyone else are different.
Enable the DM,
Offer to take some of the burden off the new DM, be the guy that keeps the party notes, takes care of the part equipment or supply list, look up rules before hand and have them ready for the DM(there is a water section coming up let me look up the water combat rules and have them ready). Help build atmosphere, be the guy that rolls to identify the monster, be the guy who role plays catching food or making dinner, be the guy who asks what the room or furniture looks like.
Help keep the party on the rails. It is a new DM running a published adventure they are not going to be ready for much improv so gently guide the players back when they wander to far off the rails. Be the guy who asks the DM out loud what would my character know about this, which lets the DM feed the party information. Be the guy who asks the bartender for rumors or asks an NPC how things have been in the town or if they can recommend an establishment.
Don't try to preempt an encounter, but if the party spends ten minutes obsessing of trying to find a secret door you know isn't there, be the guy who does something else so the DM does not have to drop hints or random encounters to get them moving.
You can go even further depending on how serious your group is. Play the example of what not to do. I once played a character in a short adventure (dungeon) I helped built and never got to use, I talked with the DM and played a character who had been cursed to die a never ending series of horrible death every day, he stumbled in to every death trap, (touching the floating black ball, running across the frictionless floor, falling in the chasm of ooze) letting the DM showcase the dangers of every rooms perils. He died at least a dozen times in three sessions, always popping back into existence an hour later, a little less stable and more pissed off each time. It was a fun house with lot of instant death traps, very first edition feel. On the one day he managed to survive without getting killed once, a lot of lucky rolls) at midnight he exploded into bloody salsa injuring another character. He had a bit of death wish but didn't want to get the other characters killed.