20Kingdom cards is rather limiting, what about not taking all the treasures with you? Easy space for three more Kingdom cards atleast. Or clothes? You could really skip on those two to get some more Dominion cards in.
But if you're restricted to 20 Dominion cards: All of Empires is the first solution. These cards may no bore you after playing with them over and over, also Events and Landmarks have a pretty good space/entertainment ratio compared to Kingdom cards. Pen and Paper replaces all tokens easily.
If you don't have Empires yet, here is an alternative solution:
- Some Adventures events for said reasons.
-Throne Room, Golem, Apothecary, Menagerie, Jester, Herald, Transmogrify, Catacombs, Village, Caravan, Upgrade, Wharf, Forager, Stables, Advisor, Border Village, Haunted Woods, Grand Market, Fairgrounds