I hate to miss out on something but just really didn't know what to do there
Then don't do anything. Find yourself a beach-side cantina, sit there drinking beer or margaritas and people-watch. Order food when you're hungry and just do that.
Another way to do nothing: wander up and down the streets looking at the shops - both in and out of the "tourist area".
How would we go about a double-decker bus tour or maybe getting around on foot by ourselves?
Someone who's done the bus can tell you more details, but if it were me: I'd get into town and ask one of the excursion hawks where I'd find it.
The way you do the "on foot" is you get to town and start walking.
This is only our second cruise and still a bit nervous about doing things on our own.
Understandable. Foreign countries can be intimidating. The way to overcome that is by doing something on your own. Doesn't need to be much. Just something. Find that cantina. Go to the excursion hawk and randomly pick something short and cheap and do it.
SIG: If you're going to create the drama, at least supply the popcorn.