Just recently as a 1st time cruiser here is what I found out since I had similiar questions.
I booked a few excursions before cruising and not through Carnival. I had seen this advice on these boards and found the same types usually for much cheaper. Just remember "ship time" vs "port time" when going this route. I know my wife and I saved a few hundred $ doing this. There are companies online that you can book through, which many people on this board have used with success.
"all-inclusive" usually meant standard food areas and basic drinks. We don't drink so no bar bill for us. The soda plan is not bad if you really want to drink a lot of soda. I found the lemonade and juices were plenty enough that next time I wouldn't be paying the $60. Special activities cost extra like bingo, certain contests and so on. We only got room service once and I tipped $2 for each of us.
No clue on the travel agent. Wise or not, Cruisemates was our travel agent in a sense and we seemed to do fine.
I wouldn't pay for a "beach excursion" unless it is something very different. Depending on the ports of call. It is very likely you will stop at a port that has a Carnival established beach. In our case in was in Rotaan. We walked off the ship and a 5 minute walk we were on the beach. We didn't pay a dime extra. But we saw $60-70 excursions for covered chairs and reserved spaces. After being there it seemed kinda of silly.