So you're giving the PCs too much to do. What do you do when they can't decide what to do next?
You've got a few options to help keep the party on track and keep your players engaged and actually playing the game.
1. Let the PCs champion their chosen path. If you're doing your job as a GM by mining PC descriptions for bait they can't resist, at least one of the PCs should care passionately about one of the adventure seeds presented and this option will happen automatically. It helps when several PCs have different motivations which make them want to do different tasks. The best in-character playing I've witnessed involved debating what the party should do next. People are mostly reasonable, players and PCs are no exception. Usually the PCs can make a deal to stick together and choose one thing to do next. If they can't work it out in character, move on to a different tactic once their heels start digging in.
2. Bring in a trusted NPC guide. This tactic usually works at lower levels before the PCs wield earth-moving power, but even at higher levels it's nice to have a neutral third party be the voice of reason from time to time. Remember that NPCs aren't just GM mouthpieces; they have their own selfish motivations which can get the PCs in trouble. Powerful NPCs like political leaders and extraplanar messengers can effectively force the PCs to do something through bribery or threats, but even refusing a mandatory request and dealing with the consequences can springboard into a great adventure as the party makes a mad dash for the border just ahead of the Duke's Own Lancers.
3. Split the Party. I've done this on occasion and rarely had a problem with it. Yes, it means some players will not play for a time, but they'll be fine if you give them enough to think about or just let them know what's happening. Best advice here: keep it short and handwave some of the mechanics. I allow only one die roll per mini-party member and then focus on a different group of PCs, even if that leaves them hanging in the middle of combat. Usually I limit myself to one round of combat before resolution or one skill roll to reflect a bunch of smaller tasks just to keep things moving and get the party reunited faster. Splitting the party lets you give each PC group a puzzle piece worth of information that forces everyone to work together once they get back together if they want to figure out what's going on.
4. Hire subcontractors. Your PCs aren't the only adventuring group in your world, are they? Outsourcing exists in fantasy worlds too, especially when pressed for time dealing with multiple "must do this right now" adventures. After all, didn't the party get at least one adventure this way when they were starting out? Introducing another adventuring party opens the door for some wonderful interactions. Will they work on a friendly rivalry or start competing for the same quests? If nothing else, this gives you some great rumor fodder, especially if the rival group wins the Bard race and ends up with better marketing. Potential clients can lament that they're settling for second best with the PC group because the Legion of Song can't take time away from saving the world.
Always remember to make the PCs the center of the story. If they don't feel engaged and able to control the world around them, they won't have nearly as much fun playing second fiddle to the powerful NPC who gets to change the world while they watch. Use NPCs sparingly, and don't let an NPC push the party around too much or they'll start to chafe under the yoke.