I a now 48, but when I was in junior high, a girl and I would exchange "orders" that we had to do, then report back to the other person about what we liked and didn't. In this way we figured out a lot of our kinks/limits.
You said:
" He's always full of great ideas and very imaginative, meanwhile it's hard for me to plan very well. Can someone give me ideas on what dares/tasks he could do as well as some ideas I can surprise him with?"
I think the important parts of what you said are that he has great imaginative ideas and that you want to "GIVE ME" ideas. I understand that you said that you are more submissive, but that doesn't mean you don't have great ideas. I am also a switch who started mostly as a sub, and one of the hardest parts of having good ideas was accepting that I WAS having ideas. Next time you do one of his tasks, pay close attention to your thoughts. You probably have thoughts like "I'm glad he didn't make me do X" or "What if he made me do X?" or maybe "OH what about X?". If you do think things like this, they probably come and go quickly. Maybe keep a pad and jot down quick ideas.
The next important part if you DO have these ideas is trying them. Originally I would have ideas that I didn't think were very good, so I wouldn't share them. But once I did share some, the other person would give me feed back. "I liked N, but Y did nothing for me", or "I hated Z." This lets you know variations of N are probably going to be good, Y is probably not the best options WITH THIS PARTNER, and Z would be best as a punishment.
Most importantly, is that these are YOUR ideas!! I might be able to give you "better" ideas for tasks for him, but they are MY ideas. There are literally hundreds of places he can get ideas from other people (including this one) but he is asking YOU for them. So don't worry much about how "good" your ideas are. Maybe his ideas are only original to you because they are different than what you think of. My advice is just trust yourself and talk to him. You may surprise yourself how much he enjoys doing tasks because they ARE from YOU.