Hi Larissa and thanks for your question about container plants. This is certainly a tough subject, because containers, especially dark ones, tend to heat up in the sun and not many plants can take the high heat on their roots. 

I am going to suggest that you figure out some kind of shade for the pots (and maybe the plants) on those hot July and August days, especially if you can't be there to water them. That being said, succulents and cacti are your best bets for hot summers and cold winters, as they were developed from desert plants that had to cope with those very conditions.

There are lots of places to get great succulents and they come in so many shapes and sizes I'm sure you will have a lot of fun choosing the ones that appeal to you. They will come with instructions about how much water they need, so find some that fit your habits. I myself love the Dragon's Blood Sedum which actually changes color with the temperature!

Have fun with your planters,