There was a mega church near us that was almost hit after the collection on a sunday morning. They have 7200 seats, and 3 services, so that can add up to quite a lot of money. I mean, they pass around buckets, so they're still getting a ton of cash. (The police got wind of it and arrested the guys before they could do anything) So throwing out some ideas, you could make it seem like a straight up robbery, but they are after something else, something a lot bigger. That way, the goal and the stakes start out small and grow bigger and bigger as the story progresses. That also allows you to throw in some fun twists and turns, and keeps the reader guessing.
For example, imagine there's an old forgotten tunnel running between the church and a hotel where the president is going to be addressing some people. The hotel is locked down completely. There is no way anyone is getting in there, unless they go through this forgotten tunnel. These guys know that the entrance to this tunnel is directly under the church vault. Their plan is to bring in the armored vehicles at the beginning of the third service to get into the vault, and from there cut through into the old tunnel with blow torches. Because it's in a built up area, they have to be quiet, and they only have an hour after the offering is collected and before the sermon ends. If one person gets wind of it, someone will alert the police. So they cannot take any chances. When they get there, they take down the security guards, and later realise that one of them is unaccounted for. They follow a trail of blood into the sunday school buildings.
Now all of a sudden, things are getting out of hand. The bad guys suddenly have to lock down the building, but it's okay, because it's still easy enough to contain. There are a lot of kids but only a handful of teachers. Your protagonist is a sunday school teacher that was collecting some supplies out of one of the offices at the time, so they don't know about him. The church is busy doing extensions at the back of the building, so there are lots of half finished areas he can move between. He tries to get a message out, creating more problems.
He is in love with one of the other sunday school teachers- a single mother, who isn't all that interested in him. You can follow her story as well as his. There has to be some kind of emergency in there, but obviously they cant send for any help because the place is locked down, so she's dealing with the emergency to the best of her abilities with the limited resources that they have. She gets word to your protagonist, who can get her some of the supplies she needs, but this causes other problems. Eventually word gets out, things get out of hand and SWAT is called in. The bad guys have to then lock the whole place down until the first group are through. Those who are left behind hope to keep the police busy at the church and then eventually also escape through the tunnel, at the same time, they can't allow the police to realise what is happening, so they make it look like they are terrorists or a group that hates Christians. They plant bombs around the building, but your hero is making life really difficult for them. At the same time, he's just a normal, nice guy, so it's really difficult for him to kill these people, but he does what he has to. (At this stage, the reader doesn't know what is happening yet either. They've gone from thinking it's a robbery to a terror plot) During the chaos the bad guys take the love interest's kid as a hostage. Your hero discovers the truth about the assassination. He can't get a message to the cops, but the love interest can (make it dangerous for her) while he goes after the bad guys to save the child, and ultimately the president. Because of his bravery, he ends up with the love interest in the end.