Have you tried making some videos together? That's fun.
Also this is going to sound weird but I don't care ;)
It's fun to go somewhere like the bar. Sit apart and let guys hit on you and try to pick you up while your boyfriend just takes it in. Then after a while just excuse yourself and leave together. I know it's a huge turn on for me when I see other men trying to get with my girlfriend and knowing they can't.
I liked Doll's answer, it most closely reminded me of my own solution.
Obviously your concerns will be predicated by two things:
1. what are the bounds you have already pushed;
2. is it you or him that is looking to keep the spark alive (and this is the big one)
I've been married 21 tumultuous years but still shag the wife at least 5 time a week. For me it is visual, so do NOT insult the effect of sexy attire (its not always lingerie), for her its emotional almost spiritual. I've told my wife what keeps me interested and, bless her, she tries. What I do is along the lines of what Doll said, for example, after a formal dinner one night a few years back I dragged the wife out into the car park and we 'dogged' - the memory of that night lasted for months. On another occasion during an intense thunder & lightening storm I pulled over in the car and gave her a seeing to - totally unexpected.
Just shake things up once in a while. Feed his interest with elements of your fantasies and you should both see the spark stay very much alive.
"Whoa, lady, I only speak two languages, English and bad English." - Korben Dallas, from The Fifth Element
"If history repeats itself, and the unexpected always happens, how incapable must man be of learning from experience?" - George Bernard Shaw
Oops, sorry Cherry. You're right, my answer was a tad one dimensional...mostly because this is a 'sex' site so I presume the advice re 'keeping the spark' was strictly for between the sheets! Let me try again from a non-sexual perspective.
If it's the "butterflies" in the tummy feeling you want I can only share with you what my missus told me. Short version: Humour.
The more you get to know someone the more 'comfortable' things get and the butterflies settle down (mostly), however, if your partner knows how to make you laugh AND can laugh at himself then it seems highly likely the butterflies will remain. I'm not talking slap-stick comedy here, just an ability to find humour in a situation when it is needed, and not get all bent out of shape if the joke is on him........this, apparently, is quite endearing and adds a soft touch to the 'macho' image. That should keep your spark bright.
If it is 'his' spark you are concerned about....well, that's a toughie. There is a saying (oft discussed amongst men) that "A woman marries a man hoping he will change, whilst a man marries a woman hoping that she will NOT." Whatever it was that attracted him to you is probably what will keep that spark alive....it has for me.
"Whoa, lady, I only speak two languages, English and bad English." - Korben Dallas, from The Fifth Element
"If history repeats itself, and the unexpected always happens, how incapable must man be of learning from experience?" - George Bernard Shaw
What my wife and I do. Is she will surprise me with a sexy dance and routine.. I like to surprise her with a secret weekend away some place romantic. We also try to act out each other fantasy. The truth is you have to be creative and responceive to each other needs. We have been together for 12 years. She is the woman for me with out any question... One other thing us men like is you ladies dressing real sexy and going out. We love to see all the other men starring at you, it gets hot and you will be rewarded tenfold later that night or any place we can get you to ourselves...
Hey! There is one solution. Why don't you both spend some quality time with each other. If you realize the need of each other, then you can give your relationship that necessary spark.
Think you were and are a perfect match making couple and thus have to live up to those expectations.
Also, go for a romantic movie together. If not the least, keep aside your egos and tell your loved one "I love you" both in the morning and while going to bed. That does make a difference. Believe me.
Try to grow together, and not apart.