There are several daytrips available and tons of tour shops around so no need to book any of these in advance, they'll find tours easily. Consider PhangNga Bay (James Bond Is), also Koh PhiPhi, also Coral Island. There are jungle tours and elephant rides through the jungle. Again all these things will be advertised everywhere. There's no need to plan all this in advance, just wait until they arrive and look at brouchers and photos and see what appeals. They could also rent a Suzuki for the day and drive around the island, there's some remarkable scenery around Phuket's coastlines. It's also worthwhile going into Phuket town and looking around the streets & shops. Phuket's most impressive temple is Wat Chalong, also well worth a visit. Don't be tempted to buy too many things from the street stalls & shops because they're skinning tourists alive with their prices. Go to Tesco Lotus and pay far lower prices just like Thais do. In 10 days I'd be inclined to do trips maybe on 5 days and spend the remaining 5 days just chilling around the resort, I'd suggest maybe alternating, 1 day touring, next day reading a book by the pool. I hope they have a great time.