We have never had a "service" for thinking day, and it wouldn't be the first thing I thought of. We used to do a district thinking day evening with crafts, food etc. Our district though is now too big to all meet & do stuff, so the units that meet where we do tend to do something. (Generally cos I organise it!) We involve Rainbows, Brownies & Guides, and this year one of our new leaders is going to organise it for her leadership book. We usually base it around an international theme, sometimes using the WAGGGS or other resources, otherwise just doing our own thing with a bit of guiding history thrown in, and always food! If you want to award a badge for attendance & are not doing a pack then there is a spiral thinking day badge, or WAGGGS do them as well.