I have been to Cancun and Riviera Maya a total of six times.
Don't worry about speaking Spanish. There is a saying that more people speak English in Cancun and Riviera Maya than in Miami. I learned a while ago that I don't start speaking what I remember from my high school Spanish at first, because they will think you speak fluently and start rattling off conversational Spanish.
Excursions can be booked at the resort. That is the best choice, especially for first timers. If you want to visit the shopping malls, you can just jump on a bus for a few pesos. Be sure you hang on to the railing ASAP. The drivers take off really fast. The malls are just like the ones in the USA.
As far as pesos, I used to convert to pesos all the time but I stopped when the locals told me it didn't matter to them. I now bring dollars in singles for tips at the resorts. I brought down $300 dollars, but I only used have of that. You can buy souvenirs or pay for excursions with a credit card. Your credit card company will convert pesos to dollars on your statement. If you really want pesos you can make withdrawals at an ATM in the resort. Just remember, you have to withdraw in Pesos, it won't show dollars. A peso is equal to about 8 cents. So if you withdraw 1000 pesos; 1000 pesos X .08 cents is $80. That's the amount taken out of your account. A lot of places will show the prices in pesos and USD, but if just in pesos, just figure the dollar amount will be 80% of that number.