Apps for organizing daily activies + "to do's"

There is Taska, Organizer To-Do: The Complete Personal Planner or Things for iPad + many more if you search

I haven't tested it out myself, but I think this one might do what you need

http://webis.net/wp/pocket-informant-pro/

I downloaded it a while back but haven't gotten around to trying it out.

I do social media for a TV show, the producer and I use Flow to stay on track. http://www.getflow.com

There is a free one we tried too, but I didn't like it. Flow has a web interface and a Mac app too.

Free one: Orchestra

RTM. Just access through any device. Believe me , I've gone through many systems over the years: Things, omnifocus, reminders, todo + a few more. Nothing is nearly as seemless and powerful as a RTM (pro account for sync) + the proper customisations for how you work. Things is also very seemless and polished (sync is an par with RTM too), but the developer is notoriously lazy and extremely slow to bring out new features (not to mention the mac elitist culture, i.e you're screwed if you want to acess your to do lists on windows or android). RTM is simply much much more powerful with the same level of polish and efficiency as Things.

Edit: misread your post, and seems that you need Calander integration. In that case google Calander w/ tasks is a good option. RTM also has Calander view for upcomming tasks for the next 7 days, so I use it for both my Calander and to do list (probably not ideal for everyone though)