Half Dome is an incredible hike, but yes, extremely challenging. If you don't feel up for that, Vernal Falls (1000 ft) is a great first step, then continuing on the Mist trail to Nevada Falls (2000 ft) if you still have energy. The trail is mostly switchbacks and stair-like rocks, so it can be tiring.
A smaller, more relaxing hike is Yosemite Falls. Lower Yosemite Falls is only a mile and 50 ft of elevation gain, but there's beautiful views and it's easy to just enjoy the falls for a while. IMO Yosemite Falls is far more beautiful than Vernal or Nevada. You can also continue to Columbia Rock, which is about 1,000 ft gain and 2 miles roundtrip.
With your limited time, I'd say that there's more than enough to see in Yosemite Valley -- don't worry about Toloumne.
Hope you enjoy Yosemite -- it's one of my favorite places in the world. And try to aim for Half Dome, even if it takes you the entire day
