If you missed Seedot and Shroomish, the next other fully evolved Grass types are Ludicolo, Vileplume, Bellossom, Cacturne, Cradily, and Tropius. Of these, Lotad is probably the easiest to get, and Ludicolo gets good moves like Ice Beam and Giga Drain. Its main trouble is that Water Stones are a bit hard to come by.
Tropius comes later, but it's one of the best HM slaves in the game. It's okay at battling, but it shines as one of the most consistent utility Pokemon you can use. Ludicolo does well with HMs too, but its better suited to battling.
In terms of encounters you can actually guarantee in a Nuzlocke, Cradily is the safest bet. It's not amazing, with terrible offenses and an equally bad level-up movepool, but its defenses and decent typing make it a fine tank if you need to heal other Pokemon in battle.
