Looking for software to track customer interactions

I am a consultant and have a database of around 50-70 contacts, comprising clients and prospects. I am looking for a better way to track when and how I last contacted them.

At the moment, I use an Excel spreadsheet with the contact names down the y axis, and months across the x axis. Each cell contains the business development activity (if any) with the contact that month, and just by looking at the spreadsheet I can get an overall picture of the level of activity with all of my contacts. But the spreadsheet is getting cumbersome and bloated.

Can anyone recommend any better solutions please? I want something really simple. Ideally standalone software.