I just started DM-ing my next major campaign for my friends, a Spelljammer campaign, and I've worked with my players to create a game plan for how leveling up will work.
Instead of the traditional milestone or experience point level ups, I'm working with my players to come up with a level up "task list" that they need to complete in order to progress to the next level. Once players complete their Task List, they can then choose when their character levels up.
For my spellcasters, I've come up with a skill challenge that encourages them to practice their spellcasting and roleplay trying to learn spells for their upcoming levels. I'm going to attach the challenge below, and I'm going to use the Wizard for the language of the challenge. Feel free to change the check type for the class of your choice to match (ex: Performance for Bard, Religion for Cleric, Nature for Druid).
Spend downtime working on magical formulas in your spellbook.
Think of the 2 free spells you get for your level up, and take opportunities to work through their formulas in your downtime. I'll know you're ready to engage in the skill challenge when you RP working on magical formulas by seeking out arcanists, spell components, or using downtime to work exclusively on that.
In order to "crack" a magical formula you will need to complete a number of successful Arcana checks equal to the level of the spell, with a DC equal to 10+the spell's level.
For example: You'd need to succeed on one DC11 Arcana check to correctly crack the formula for a 1st level spell; You'd need to succeed on 3 DC13 Arcana checks to correctly crack the formula for a third level spell.
You can only attempt one of these Arcana checks per long rest. You do not need to complete the skill challenge on consecutive Long Rests.
You can only do this process twice per level, representing the spells you gain for your upcoming level up.
These spells become immediately available to you for your use, you don't have to wait for the level up to cast these spells.
You can also attempt to crack the formulas for higher level spells than you can typically cast, with an increasing DC represented by a modifier added to the DC equal to the number of character levels you would need before being able to normally cast the spell.
For example: If you wanted to crack the formula for a 3rd level spell as a first level character, you would need to succeed on three DC17 (10+3+4) Arcana checks over the course of at least three long rests. If you wanted to crack the formula for a 9th level spell as a first level character, you would need to succeed on nine DC35 (10+9+16) Arcana Checks over the course of at least nine long rests.
My players are really excited to try this, and I've already seen some of it come into play in our very first session. I wanted to share this with DMs and Players to encourage making leveling up a challenge unrelated to experience or milestones. This is one of the tools I'm using to do this, and I'm interested to keep feeling it out.