we've been there several times and here's what we do.
dive for the first 4 days or so, and then do the tourist / relax thing for the rest of the week. i really like dive with martin and aqua safari, but there's a million other suggestions on the coz thread.
i have not dove cenotes yet, but that is because the diving off coz is simply amazing....so why spend the time going across when i can just go for a boat dive that will make me more than happy.
the palancar sites are my favorite. make sure you hit palancar caves (not truly caves). san fransisco is my fav 2nd dive site. honestly, you just can't go wrong. i didn't like tormentos that much, but again, that's just compared to other 2nd dive sites i've seen.
i have rented scooters in the past and all i can say is that putting a wetsuit on over severe road rash on your legs, arms, and chest takes a lot of fun out of diving. rent a car! we use the place right across from the grocery store Mega. I think it's called "spend less" or something with the word "less" in it. he was super nice and gave us a reduced rate and knocked 1-2 days off our rental. we returned the car and he drove us to the airport.
mega is a good grocery store. we always stock up on basics and get a giant jug of water that lasts most of the week.
i've found that bringing starburst candy on the boat for post dive treat goes over well with the dm's, captains etc. of course, they'll have fresh fruit etc, but something to share with them goes over well. be competent with your gear assembly, be jovial and have fun with the dm's and you'll be the "teacher's pet" and have a lot of fun.
while heading out to the first dive site, you'll go by all the beaches south of town. look for the type of beach that appeals to you and ask the dm's what each beach is like. they'll tell you, "that's a tequila / party scene, that one is more chill" etc. choose the one you think you might like and then head there in the afternoon. we like san fransisco beach the best when we were there a few weeks ago. we went several times and had the same waiter. we tipped well each time and we're taken care of VERY well each day.
the breakfast place above the museum is great!
lastly, you HAVE to visit Wet Wendy's. its basically in the square downtown. we met clara and she showed us all the renovations she's been making. very great person doing very great things. and the margaritas are amazing.
if you want to spend more than a few pennies, Prima is one of my fav restaurants of all time.
that's my 2psi.