What can I do?

i think first you need to think of him and how you know him know how he loves to spend time and what he is excited about most i dont think that any one will give you the right formula , cos he is not any one else , and there is only you who know him well but i think you can start by focusing on the things he dosent like so you eliminate them and then see what he likes write things down and check what its the best , and then use your imagination :)

Can you make him a special dinner or bake him a cake? Is there someplace he's really been wanting to go that you could take him or a movie he's been wanting to see? Maybe just pack up a nice picnic with all his favorite goodies, and take him to a nice park or lake.

Some of the things I've done for my husband when we were dating were making it a point to have dinner at his favorite restaurant and ensuring that his favorite dish will be available, get him a present which he had sacrificed not getting because of other reasons which means using my money I saved to get it for him or simply get him something I think would be good or help him look great. The list goes on but I always would convince myself when I am running out of ideas that he would like whatever I have placed my heart to buy and make it happen because it is not the gift or experience but my sincere efforts that count.

My friends generally get me books and book vouchers as presents, which I always love

It depends on his interest. You should know by now what he likes and does not.