I've camped in Sequoia several times, and have stayed in Giant Forest, Grant Grove, and Cedar Grove. We've never used a professional tour, and I don't think you need to either.
You can visit the visitors center located at the south entrance to the park or the one up in Giant Forest or Grant Grove for ideas & maps on things to see.
I highly recommend visiting the Giant Forest and General Sherman tree (largest living thing in the world). In one day you can visit that, Crescent Meadow, and Moro Rock. Moro Rock is a 1/4 mile of steps leading to the top of a dome. Crescent Meadow offers a beautiful walk (not really a hike) around some meadows among the giants. Be sure to see the chimney tree and Tharpe's Log (a fallen tree turned into a cabin - it's that big).
If you go to Grant's Grove, there is a nice walk among the giants there, too, which is really nice.
When we go we always stop at Hume Lake. There is a beach there and you can swim - it is really nice. There is also a Christian camp at this lake that rents out kayaks, canoes, and row boats for a small charge. We've been renting the kayaks since my kids were 7 & 8 (somewhere around there). The beach is really nice, and it is a great place to spend a couple or few hours.
There are a couple of caves - Crystal Cave with a 1/4 mile steep hike down to the mouth of the cave. We've been on that tour a couple of times, and we've also been to Boyden Cave on the other side of the park - very easy to get to, but it's a drive.
Let me know if you have any more specific questions.
