Does anyone know about the crowds and amount of players at the practice rounds? I have a chance to go but not sure which day to try for. I was thinking of Tue as a lot of the top guys will probably chill out on Wed. Or is it the other way around?

Very little gallery. The pace of play is slow. It's most singles or twosomes. Guys stay on one green for ten minutes hitting lots of chips/pitches, bunker shots, and putts.
This is the scene Monday, Tuesday, Wednesday.

frozen rope pretty much summed it up. The crowds are pretty similar all practice round days as the USGA pretty much puts out the same number of tickets per day. But as he said, don't expect to see continous play if you are sitting at one hole, each group will take anywhere from 10 to 15 minutes putting and chipping around each green. I have been to 5 Opens and I spend the practice round days walking the course seeing a lot of the holes that will be featured in the weekend coverage, to get a good idea of where shots are going. I spend a lot of time at the driving range watching, and then usually follow my buddy (who is a tour caddie) and his guy during their practice round. The actual tournament days, I will park myself in a prime bleacher location and sit there all day watching the groups come through. It's a lot better than trying to fight the crowds along the ropes. Sat and Sunday, I'm back home watching in the comfort of AC and glorious HD !!

I was planning on spending time at both the range and putting green but mainly just wondering the course. We are going out as a group of 8 to play on sat, sun & mon north and east of San Diego then driving down to spend the day tue or we could flip days around. We fly back here to Chicago on Wed so it looks like a great guy get away. Plus one advantage of going to the practice rounds will be the ability to take lots of pictures and do some shopping at the USGA tent.

chef, sounds like a great trip indeed. And you're right to me one of the biggest advantages of the practice round is getting some great pics of the course and players. I've got some great shots from both Oakmont and Winged Foot, I didn't have my camera for Pinehurst. Enjoy the trip.