Hi Helen, Not sure if the accommodate for groups that large but I know the treetop adventure place on this website is open all year round as I have been a few times with my nephews. Theres also a watersports place I’m not 100% sure what its called but its towards estepona and they do wakeboarding and cable skiing. This would be great for a big group! Hope this helps
Mud buggying is a great activity here in Spain, there’s also the prison break activity but that’s a little further out of your looking for things to do in Marbella. What about private Yacht hire or a beach club brunch?