Kristi - we went on the Tulum tour, Isla Mujeres, ziplining and took the Palace Resorts bus into Cancun one night. We booked everything through the tour operators that have desks set up at the resort.
The Tulum trip was a wasted day in my opinion. After the 2 hours on the bus we stopped at an expensive shopping area, then back on the bus for a half hour for the rest of the way to Tulum. Once we finally got to the ruins I was so bored with our guide that we all took off for the beach. I didn't even look at what I was taking pictures of. I did the walk by, point and shoot, we'll see what turns up later. We had lots of fun in the ocean and then spent a little time at the market where the bus drops you off. Then it was another hour on the bus to Xpu-Ha Resort where the food was mediocre but you're just so starving at that point you battle the rest of the group. There was barely any fish in the snorkeling lagoon and kids in kayaks kept running into us or hitting us on the head with paddles. Back on the bus for another hour to Moon Palace.
Isla Mujeres was a lot better. Half hour bus ride into Cancun, catch the boat to Isla Mujeres which is about 45 minutes. Arrived at Dolphin Discovery and ate the buffet lunch, then grabbed the $5 taxi ride to Isla Mujeres Palace. There we lounged in the pool and at the swim up bar, tried out the water cycles, and ate again at their buffet. Then we took another $5 taxi ride back into the town for some shopping. The boat ride back is more fun with tequila shooters. Then a quick nap on the bus for the ride back to MP.
The ziplining place is across the street from MP but they take you on a 20 minute ride to try to sell you additional activities. It was fun and only lasted about half an hour. They didn't want to take us back to the resort right away so a group of us paid $28 to have a bus take us back across the street so we wouldn't have to wait.
The bus to Cancun was quiet, we got off at Beach Palace because it is close to La Isla for shopping. Don't bother going to Flamingo Mall it's just expensive crappy stuff. We walked around La Isla and then checked out Beach Palace, grabbed some dinner there and caught the bus back to MP.
I think safety can be an issue anywhere. Just consider your nearest downtown, there's probably areas you avoid and people you wouldn't talk to. It's the same thing anywhere, don't look for trouble and you'll be fine.