In regards to the weight increase, don't forget, a pint of water weighs 1.25lb, so if you're drinking plenty of water then your weight can fluctuate a little depending on how much is still in your system.
For constipation - plenty of fruit and veg. Try not to peel fruit eg. apples, if you can, as there's more fibre in the skin. Plenty of fluids. I aim to get through 1.5l of water a day (don't forget, fluids can be tea, coffee, water, fruit juice, squash etc). Also, healthy snacks like nuts are good too, as the oils help and certain nuts are quite fiberous such as almonds. If you can eat them, a bowl of almonds and dried fruit as a snack are great.
Personally I go for a big bowl of kelloggs bran flakes (just because they tend to have more fibre than shop brand ones), and milk, plus my OJ and calcium for breakfast. I'd have alpen as I think that does get through you well (sorry if TMI!), but I get indigestion from the oats
You could also get some bran or linseed and add that over your cereal in the morning. Or have a go at making bran muffins (which I may do on the weekend now I've mentioned it!).
Hope something in there helped