Canon Rebel XTi - need a good lens & filter

I found the 17-85 IS to be a pretty good all around lense.

Is there something specific you don't like about the 18-55 mm kit lens? The 17-85 mm IS is not that much optically better than the 18-55 lens, but it does have longer focal length coverage and IS, if those are what you're looking for. It is still a slow lens. If you don't need wide angle, you can also consider the 28-135 mm IS, it gives you more tele but no wide angle on your XTi. It may make more sense to buy a mid range tele to give you more versatility. The 70-200 mm f/4L costs about the same as the 17-85 mm IS lens, and is good for portraits, candid shots at a distance, and can also capture outdoor actions of your family, among many other uses. Of course it is not lightweight but it is not hefty either.
How about prime lens? These usually are lightweight and has better optics than most non-L series zoom lenses, if IQ is what you're looking for, at reduced price (compared to zoom lenses). But prime lens is not an all purpose lens.
And if you really don't want to change your lens at all, then there is the superzoom lenses. Tamron has 18-200 mm and 18-250 mm. The optics are no better and possibly not much worse than the kit lens. And both Tamron and Sigma are going to push out IS versions soon.
The UV filter is mainly used to protect the lens, but it may degrade the image quality if you buy a cheap one. The best ones are made by B+W and Heliopan, which will not degrade the image but they will not enhance the image either. I have the B+W UV filter on my expensive lenses, the 17-55 mm f/2.8 IS and the 70-200 mm f/2.8L IS, to protect them from my son's curious fingers. But these filters cost about $100 each, won't make sense to use it on the kit lens.
There are a lot of other special effect filters, like the Polarizer, graded filters, ND filters, color filters, etc. Whichever filter you buy, get a good quality one.