It's hard to see much of anything in
Disneyland from the DLH. There are so many trees. Space Mountain pops out and the back of Spalsh Mountain, I think. From the rooms on the park-view side of the Sierra tower, though, you can have a nice view of California Adventure and of Downtown Disney. You should also be able to have a nice view of the fireworks from that side, too, but probably mostly from about the 4th floor or so up, so that you're over the top of the buildings in Downtown Disney. You should know, though, that on that side of the Sierra tower, there is an elevator that runs up the outside of the building, so there are some rooms that are next door to this elevator (sorry, I wish I could help you with room numbers, but I can't) that have a partially obstructed view because the elevator is in the way. Be sure to ask about that when you check in, if you decide to ask to upgrade to an upper-level park view room.
I think there are some other locations in the other towers where you should have nice views of the fireworks, if you're there when that is going on. And, at least half the hotel has a nice view of the pool, if that's more important.