Legos are a hit here too!

Also, castles and knights, army men, Lincoln logs. If it's nice outside, drawing a chalk racetrack on the driveway and doing bikes/scooters is fun. Indoors: audio books, computer time with educational games, color by number, latch-hook, cross-stitch, independent reading time, dominoes, Geotrax, magic trick set, Sudako, etc. My oldest used to be like your ds, and has finally learned the skill of choosing what to do with free time. Hallelujah!

I pointed out to him that this was a skill to be learned - many adults aren't sure what do with their free time either - or make poor decisions about what to do. My dh told our ds that he LIVES for getting to his free time. He finishes his work as quickly as he can, so he can do the things he wants to in his free time.
I told my ds that I could pick for him each time, but that ultimately free time is a gift that each person needs to decide how best to use, so my picking for him really wouldn't let him use that gift. Sometimes, when asking him what to do, if he was acting unsure or had what I consider an "Eeyore" attitude about it, I give 3 suggestions and have him pick. If he still doesn't like those, I pick 1 thing - usually what I think he will like the
least and set the timer to have him do it for at least 20 minutes. Then, I ask again what he'd like to do - he gets the picture quite quickly that it is more fun to choose himself than have me in charge of it.
Carrie and I have our dc play each Friday, and all the boys choose get to choose 1 thing to do for 20 minutes. It has been an incredible learning experience for them. One time, my middle ds picked Geotrax. Everyone took 20 minutes to set it up for him, and then when we set the timer for the 20 minute Geotrax time, he cried because he wanted to change his mind and pick "cars". We said "no", and he spent 5 minutes of the Geotrax playtime in the corner in time out. The next time, he carefully weighed his decision more. Another time, one of the dc picked to play with 1 paper moth he'd made. We had to point out how everyone could not play this, so another choice would be better. Another time, someone picked the LONG game of Risk and when the timer rang was way upset they weren't done. The next time he picked the short version of Risk and was very happy. Yet another time (in fact this past Friday), all of them first chose to pool their 20 minutes together for 100 minutes of playing knights and castles. We pointed out this was just fine with us, but they would all be playing 1 hour and 40 minutes of this one thing then. They put their heads together and one picked play outside and another picked put on a magic show, resulting in 1 hour of knights and castles, and 40 minutes for the other things.
Learning to choose what to do for free time, while considering if it is alone time, time with siblings, the amount of time you have, the weather - and so many other things, all lead dc to master the skill of "what to do with their free time". You may want to have a rotation for your ds. 30 minutes independent play in his room, 40 minutes of playtime with his sister where they each pick something to do together for 20 minutes, 30 minutes of independent play out in the house, and then maybe add some other "have-tos" like 10 min. independent reading, 15 minutes audio books, 20 minutes outdoors, etc.
It's worth the time to work on this skill - you are right to identify this! Too many dc look to their parents to be their entertainment advisors, filling their every moment for them. Don't give in to the temptation to do this - we don't want dc who are 30 yo calling us for what to do on a free Saturday!
In Christ,
Julie