I agree with Sue. It doesn’t matter if he/she is a high paying client or not.
Keep things challenging and interesting all the time. Never let them think that because they’ve reached their current goals, it doesn’t mean there are not new and better ones to achieve. You could get them interested in joining some type of race and your part would be to help them train for it so they can perform the at their best. You could also offer to do the race or event with them, so they feel like they have a buddy to depend on or be accountable to. This has worked really well for me.