So what I thought you could do was a live stream with a give away included.
Start a forum thread, get people to suggest 5 of their favourite slots. Leave it open for a week.
Have a random wheel with the numbers of forum entries (e.g. 1- 250).
Spin the wheel and then pick 1 of their 5 game choices, could have a second wheel to determine it at random.
Play the game until you have a bonus. Once you have a bonus on that slot, have a prize giveaway (amazon voucher for example) for a win over a certain X. For example – 50x = £25 100x = £50 and 200x and above £100. Not sure how much you guys want to give away or what you budget is with regards to a giveaway, those were just an idea.
I am sure whatever you do will be great!