Pick an isotope and a date and this spreadsheet will do the rest..
Includes most popular isotopes in radiotherapy, with space to include your own. (137Cs, 60Co, 198Au, 125I, 131I, 192Ir, 103Pd, 32P, 153Sm, 90Sr, 90Y)

This excel spreadsheet is very basic but makes life easy (which lets face it, that's what it's all about).

It shows decays for 137Cs, 60Co, 198Au, 125I, 131I, 192Ir, 103Pd, 32P, 153Sm, 90Sr, 90Y

All you need to do is pick your isotope and enter the calibration date and activity. This sheet then calculates your activity today, at daily intervals from calibration, at any date of your choice and at 5&10 half lives (for disposal considerations).

There is plenty of space available to add any additional isotopes that you use frequently, although you'll have to look up the half lives for these first.