Hi,
I was in Yosemite last week (Su-Wed) so I thought I'd chime in.
We arrived late in the day on Sunday without any traffic, but it is a pain getting in on 'regular' hours, especially before noon, so I definitely second the advice to get there early. Maybe you could look into leaving the bay area on Friday and driving to the central valley (relatively cheap motels anyway). That way, if you get going around 5am on Saturday, you might avoid a long wait at the gate. We drove out of the valley Sunday night during sunset, again no traffic. On Monday, we were back in at 6am, no traffic.
Traffic on the valley floor was bad between say 10am and 5pm. Ironically, the best time to be out hiking and enjoying the valley in this weather (we had temps of 95-100F) was before 10am and after 4pm. So: get there early, get a parking spot, use the shuttles, and leave the valley after the early evening rush.
Hiking: we did panorama trail from happy isles to glacier point in something like 7 hours on Wednesday. We started at 7:30 to avoid the heat and the crowds going up mist trail. The trail was wet, we got soaked climbing up to nevada fall, but it never felt dangerous - just keep focused and be well aware that a slip anywhere could be fatal. Also: I wouldn't go down mist trail - if you're doing the hike in the opposite direction, it's a lot safer to take the JMT down.
At Illouette we saw people being careless on the granite by the stream. Please don't go near the water and the slick rocks, it looks tempting but it's very dangerous.
We got to glacier point around 2pm. If you do the hike in this direction, you can finish with icecream and a cold drink
We hiked down to the valley for a long dayhike but I wouldn't recommend it. My feet and knees weren't happy with the endless descent - I'll take climbing over descending any day... It should be easy enough to find a ride down.
Make sure you've got sun protection and enough water. For us, the heat wasn't an issue, but we did start early, and I wore long sleeves and soaked my cap and head at every trickle I saw. Also, we're both pretty experienced (if slow) hikers and backpackers, although this was our first long hike of the season.
On our other days we walked around the valley, went to Bridalveil and Yosemite Falls and visited mirror lake. I agree with Dave: you really can't go wrong on the valley floor and it is a must if you want to see yosemite. It's also a nice contrast to be on the valley floor looking up one day, and up on the rim looking down the next. Tioga Rd is going to be open this weekend, but it's still a long drive up there and the trail will be wet and possibly flooded. Check the current conditions page on the yosemite website, and/or ask at the ranger station. Last week I checked and was told that you'd need dependable navigation skills because the trail was covered in snow.
Be safe and have fun out there!
marjolijn