I would recommend two lenses, depending on how much you want to spend.
There is the Pentax 50-200 and the 55-300. I have never used the 55-300, however I have been told it is slightly superior optically to the 50-200. My DW owns the 50-200 and I have used it extensively. When I want to go light, it is one of my favorite lenses. I don't know if I just have a good copy, but I am extremely pleased that lens. There are several iterations of that lens. For your camera, I would recommend the DA version, not the DA-L version nor the WR version, unless in the future you will be upgrading to a weather proofed camera body. The DA-L version does not come with a hood nor AF quick change. The WR version is the weather resistant version and is more expensive, but since your Kx is not weather resistant there is no need for that option. The 50-200 is 3 1/4 inches closed, 5 inches extended and very light. If you can't tell, I am a fan of the 50-200. KEH.com is selling that lens in LN- condition for $99 with hood and caps. That's a steal. The new WR version is selling for $247 and the 55-300 is $397.Here are a few taken by the DW with the 50-200 on a K10D.]
Thank you so much! I was hoping to be able to find something around $100, and I so appreciate you even finding a deal for me! I will check it out, thanks again! I just feel so overwhelmed by all of the terminology!