Hi,
I would recommend taking care with returning to activities which caused the initial disc protrusion / disc bulge in the first place (e.g. sitting, bending, lifting activities) as these activities are likely to place tension on healing scar tissue and if excessive, may result in tearing of scar tissue (rather than remodelling) and therefore recurrence of the initial injury even following discectomy. I would also usually advocate pain free McKenzie based extension exercises to restore full extension ROM. Lateral flexion, rotation and flexion ROM exercises can usually be added gradually and provided they are pain free. Progressive core stability exercises can usually be added with emphasis on TA, Multifidus / ES. A functional restoration program is important to retrain the back and core stability during functional movements. Hydrotherapy / swimming can also be beneficial as well as building up general cardio exercises (e.g. walking / swimming). I wouldn't rush back into abdo crunch / sit ups - unlikely to be of major clinical benefit - i would initially focus more on neutral core exercises e.g. prone hold / bridge hold / side hold etc. Although if you really wanted to these could generally be added gradually provided there is no increase in symptoms. Here are some ideas for: lower back strengthening exercises. Use your clinical judgement to determine which ones are indicated.
Hope that helps