Hey Gordo,
My 2c
Sunrise over the canyon is quite nice. If you want the mule rides down / rafting then I suspect they will be well booked out months in advance. Oh yeah, it was cold / snow on the ground in March. Jill and I drove from Vegas to the canyon one arvo and got in at night and then had to leave the next arvo. I would suggest at least a full day to get in some walks.
2 nights is all you need. The themed casinos are insane. There are a lot of shows on - singers, Circus de Sol, etc. If you know your dates, look on the web for whats on and buy tickets before you go. Any tickets half affordable always seems massively sold out when I have been there. Wear comfortable shoes...the "strip" is long and the casinos are huge.
No idea
The bridge and the hilly bits near the harbour with the cray zigzag street, and street cars, plus Alcatraz are all worth seeing. For something different, I have done a kyak tour of crazy house boats moored in the harbour north side of the bridge but might be a bit cool when you are there.
No idea but apparently you can rent these barrels....
cheers
Matt
It is America's home town ya know 
On a more serious note we do have Hannibal Rocks here With trail riding, rock crawling events and rock racing events, not to bad for a little hick town 
