For "free" minutes, it would always be on the Level 1 beds. But they could pay, per session, to upgrade to better units (and should be encouraged to do so).
That said, if there is anyone still doing "minutes" you should do a search here on pricing and why minutes is the worst possible way to do your pricing.
That said, the same idea of the lotion special can be used. This has been discussed here as "Din's trick" (if you want to search). Basically, if they buy a $50 bottle of lotion, they get $50 to use on SINGLE SESSIONS.
NOT $50 toward a "package"
NOT $50 toward upgrades
But $50 toward single session walk-in pricing. So if your level 1 walk-in price was $7, they could get up to 7 free Level 1 sessions ($7 x 7 = $49). Obviously your GOAL would be to have them use that toward your more EXPENSIVE units - e.g. $35 for a walk-in high pressure or $25 toward a
spray tan or $16 toward your biggest VHR bed, etc.