I had Girl Scouts that age. At eight, most of them were up to stringing largish beads, decorating boxes. BUT they also had an attention span of about fifteen minutes - so you are looking for a fifteen minute craft. Candles and soap were good. Their attention span for making pom poms lasted for weeks - they LOVED that. Getting those photo boxes (or paper mache boxes from the craft store) and stuff to decorate them and having them make their own treasure boxes is good. Having them decorate their own cupcakes can take fifteen minutes and provide ENDLESS entertainment.
What they really wanted to do at a sleepover though was watch girly movies (The Princess Diaries was a big hit at that age), have a dance party in the basement, and gossip about school. I’m just finishing a sleepover for my now twelve year old - we went out to the theatre to see a musical instead of watching movies, and then they gossiped, played board games, gossiped some more.
I found at that age picking a theme worked well - we did a Luau at about eight. So they strung flowers into leis, had pineapple juice with ham for dinner, and learned to “hula.” Last year, when my daughter turned eleven, we had a mystery party - that worked really well with eleven year olds - but eight is probably a little young.
ETA: Plan ahead for the mess - we had great fun making paper at about nine or so - but MESS! Same with decorating cupcakes - great fun, but you’ll be cleaning frosting off the ceiling.