My best piece of advice to keep it fun is not to over do it. When your horse is comfortable at a height, stay at it for 3 more training sessions then bump it up. Too many horses are screwed up by jumping too high too fast! I also love running courses over ground poles instead of jumps to practice "courses" without overworking the horse.
Also, few horses really need to "practice" jumping a lot. After a horse's confidence is built up, and they "get" striding, I rarely jump them. I used to show 3' to 3' 6" hunters/jumpers/eq. We only ever jumped maybe 2x a week? We did A LOT of ground pole courses though! We rarely ever did the height we did at shows at home, however, I was blessed with a mare that could jump the moon and she knew she could do it, and she would often seriously consider jumping out of the ring, you had to make it VERY clear every time you came to a corner at a canter that was NOT what you wanted. I almost think she looked forward to show day, because then she could really GO!
With my mini, I jump only the week before show, and maybe a time or two in between shows - I swear by ground pole courses! He knows his job, he loves that job, I want to keep it fresh and new! He knows what he's doing, no sense in drilling it into his little skull!